Maokai is a relatively strong AP tank.
Edits- 17/2/2011 -Switched Randuin's with Thornmail as main armour item.
-Added Frozen Heart in for last item.
20/3/2011 -Started playing Maokai again in an attempt to master him;
-Changed Exhaust to Ignite
-Changed Masteries
-Changed Items
-Added more SPell info/opinions

Q-Arcane Smash- While this won't be your main skill in the laning phase, it is still very useful with some CDR. I usually snag at least one level of this at level 4 or 5 simply for the knock-back. The knock back can be used similar to Xin's, except that you can use it every 5 seconds rather than 10. In large team fights, this will be your bread and butter spell. This is also your most reliable escape ability, though it is not a very good one. Just cast it behind you, which knocks back the people chasing you and stuns them.
Arcane Smash is: A good CC mechanism and disruption spell
An escape tactic for both you and your teammates
Pretty okay for farming is your Sapling doesn't kill the minions in one go.
Arcane Smash is not: A early game spell
A damage dealing spell
As powerful as Xin's knock up, so don't think I'm suggesting this
To be used right after Twisted Advance because it will interrupt the root.

W-Twisted Advance- This spell can be either very powerful or get you killed very often. Uses:
1. Your combo. This combo is great for nuke damage but should be used cautiously. Make sure no ganks are nearby via Sapling, then go for it. Click on the target, Advance toward them, toss a Sapling on top of them, then wait for the effect to end (keep an eye on the debuff under your targets portrait). Next Arcane Smash them, either towards your team if they're nearby or away from you if not. If your team is not nearby or the guy you just comboed still has some health, run like hell, Arcane Smashing and Sapling tossing as you go.
2. In a team fight, you'll want to target non-ranged champions or champions with an ability you don't want them to use, i.e. Tryndamere or Nunu.
3. The final use is as a very situational escape tool. You can teleport to enemy minions or neutral monsters. It may seem silly to your teammates, but leaving some enemy minions alive before beginning a fight will be a nice tool in your favor.
Twisted Advance is: A root plus a silence
A great initiator/combo when combined with Arcane Smash and Sapling Toss
Twisted Advance is not: A stun (you can still get chewed up by champions like Ashe)
A high damage spell
A escape tool (not consistently)
A good idea if it gets you in the middle of a angry enemy team

E-Sapling Toss: This is the spell that sets Maokai apart from other champions and makes him a master of utility. As a matter of fact, it screams �I CAN DO ALL SORTS OF COOL ****!�
Spell Functions as Follows:
1. About 0.5 sec. Travel time
2. Impact burst +0.4% AP
3. Sapling has a delay between impact and chase, about 0.3 or 0.4 seconds
4. Sapling chases, note that it has to accelerate up to full speed, though this is not really noticeable. Another Note: Actually, I think it might simply speed up as the target gets further away, not accelerate. This needs some more testing.
5. Sapling explodes on impact with first object or explodes after 2 or 2.5-ish seconds seconds. If the target dies before the Sapling can hit it, the Sapling will simply hop in place and then detonate. Note that the Sapling's blast is pretty big; the edge of it often is what will hit a fleeing champion.
If you want to ensure that the target gets hit by the full 100% AP stack from the Sapling, root them first. Just remember that you do lack escape capabilities, so be careful. The burst damage from a single Sapling is pretty powerful, and will, naturally be your number-one harassing tool early, mid and late game. In the event that the guy you're trying to hit can outrun or dodge the Sapling after the initial burst, just toss Saplings so that they hit him with the Sapling's explosion rather that the landing nuke, since X+60% AP is better than Y+40% AP. Furthermore, you should never get ganked because of its somewhat over-looked ability as a ward. Sapling Toss is also your first and foremost farming tool. It is, however, a somewhat costly farming tool, which is why we'll get mana regen early game.
Sapling Toss is: A free ward. Always have one in the bush if you see someone MIA on the map.
A very high damage nuke
A farming tool
A scouting tool
A last-hitting behind turrets or over bushes tool
Sapling Toss is not: Anything it was not intended to be, I guess.

R-Vengeful Maelstrom- This ability has such potential to be awesome that it frustrates me to no end. The main issue is the delay, which may, or may not, be a bug. They did shorten the casting time so it is now not so much of an issue, however, the spell still has a small percent chance of not being cast at all. I have had many times where I'll have this spell cast under an entire enemy team, mashing the R as I run away to no effect whatsoever. So, since this is hopefully a bug, I'll just treat it like it functions normally.
You should cast this spell:
1. In the event of a team fight of any size.
2. If you have a nearby teammate getting chased at low health, cast this in the expected chase route of the other team. That 20% damage reduction can keep you alive at 25% health.
The number one thing to remember with this spell is that it is defensive. You're going to want to cast it under you teammates, not the enemy team. Ranged damage, so long as your teammates are standing on in your shield radius, is reduced.
You should turn off the spell:
1. If both teams are fleeing or your team is chasing to hit the enemy for that last smack of damage.
2. If a enemy is fleeing at low health. Be careful about this; if your team needs the defense more than the kill, leave it on. If your team is fleeing, go ahead and turn it off. If more than 1 enemy is fleeing, you do have more of an incentive to turn the spell off.
3. If no one is within the circle, of course. However, you should have hit your target as they were fleeing as dictated by #1.
Now, if you do really want to do damage with this spell, this build does give you enough CDR so that when VM is at level 3, you can cast it every 16 seconds, give or take. I still do not really recommend doing this, since it would be better to have this off cooldown in the event of a gank.
(If I forgot anything, please comment)
Vengeful Maelstrom is: A massive defensive buff for your team
A way to save your teammates at low health
A low damage area-effect nuke
A way to reduce damage so that you have a larger chance to escape a gank
Vengeful Maelstrom is not: A high damage area-effect nuke
A way to last hit during team fights
A damage dealing spell. Again, the main use of this spell is to turn fights in your favor and save teammates.

So, Items. Now, Maokai is depicted as a support tank, and you can build him as such. The above build is my standard build for Solo play. The following build will feature some alternate items according to the situation at hand (shown in blue).

1. MEKI PENDANT- Pretty self explanatory. While Maokai is not particularly mana-hungry, a Meki pendant will help you keep the hurt on for a longer period on time.
The alternate to this item would be the MANA MANIPULATOR, which you can build into a SOUL SHROUD later on. Soul Shroud is used in the event that you think your team may need some extra health and/or are incompetent.

2. AMPLIFYING TOME- Welp, this little guy puts moar "OOMPH" into your attacks, and will be built into FIENDISH CODEX later. If you are building Soul Shroud, now would be the time to get a RUBY CRYSTAL

3. FIENDISH CODEX- Mana Regen and AP, what more could you ask for? This should help eliminate any mana issues you had early game. IF you were building Soul Shroud, here your should get a KINDLEGEM using you Ruby Crystal.

4. MERCURY TREADS- If you've been farming with your E+Q combo, you should be able to get these in one go. If not, pick them up bit by bit. I almost always pick Merc Treads even when I play other champs like Urgot because of the Magic Resist and CC reduction, but you can pick any boots you like.

5. Alright. Now that we're getting into the mid-game, its time to start tanking up. The next item is BANSHEE'S VEIL. Pretty straightforward, you're going to need it's shield and the mana it gives you. The mana gained will completely take care of any mana issues you had previously.

6. And now it's time for MORELLO'S EBIL TOME- This item was, more or less, made exclusively for Maokai. You get Mana Regen, CDR, and 75 AP all in one slick, Chinese-looking package. And, of course, if you're building for that Soul Shroud, you should now get SOUL SHROUD.


7. Now, there are three items you can pick here, all of which are situational. First off is RANDUIN'S OMEN. Randuin's is a little hard to place, but I recommend getting it against AP casters or other various champs. MAKE SURE YOU USE IT'S ACTIVE. Randuin's active goes hand-in-hand with Maokai's ult and can deal some ***-tastic damage if handled well. Secondly, THORNMAIL gives you ***-loads of armour and that sweet passive, plus its plant-themed! Whoo! Anyways, pick this up in the event that you're fighting a Warwick, Ashe, or other Auto-Attack carry. Secondary choice number two is SUNFIRE CAPE. I've never been a fan of this thing, but it builds well with tanks. Pick this up if you are fighting other AP carries or low attack-speed champs.

8. Now its time to build a attack item. ABYSSAL SCEPTER works well with a close-range caster like Maokai. Plus you get some extra Magic resistance, which you can never have enough of.

9. By now the game should be over. If not, get GUARDIAN ANGEL in the event that you get focused often. Now, I usually go with AEGIS OF THE LEGION since it helps keep my teammates alive and, according to LoL wiki, is worth the money. ADDED: After testing, I noticed that picking up FROZEN HEART late game helps just as well as Aegis. It's up to you, but I still usually end up getting Aegis.

And that's the build!

Maokai is, from what I understand, babby's first AP tank. He's got two stuns, a slow, a reeeeeeeeeeaaaaaaally long-range hard-to-dodge bomb, and a kind of confusing ult. Anyways, here it goes:

Summoner Spells- Ghost is just great for everything.
Exhaust- Use this on a AD carry in a team fight or to keep champs from escaping the Circle of Life (tm)

Farming- Maokai's auto-attack does very solid damage; much more than Galio or Singed. Use this to last hit early game alongside Q and Sapling Toss as needed. Later on, toss a Sapling onto a horde/flock/herd of creeps, use your Q to drain their health to near 0, and then collect your Jew-gold. You should be farming throughout most of the game with this combo to collect as much Jew-gold as possible to turn into items so you can make your enemies give you Jew-gold.

One-on-One- In a single enemy fight Maokai behaves a good bit like Nasus. First stun them with Twisted Advance (w), then toss a sapling on their head or in the direction where they may be running. Use Exhaust as needed. Next, turn on your R and use Q to slow them down. At this point, just beat on their head and use your W+Q combo as needed. Toss saplings BEHIND your target so that they are guaranteed to hit them (what I mean by this is to place the targeting rectacle slightly behind them, but still where you will hit them with the initial burst). Use Ghost to chase them if they run away, and make sure you deactivate R if they ever look like they may retreat.

Team Fight Tactics- Number one is to use that Ult to direct your team. If you're going tower diving, drop it around the tower and push. If you're in mid, slap it down in front of you and go for it. Just remember that his ult is not for the damage; its for the protection. If a enemy starts to try to flee the Circle of Life (tm), just jam R for damage and then try to stun or Q them so they don't get away. As for your other skills, W+Q is a standard, and make sure you toss Pikmin BEHIND a target if possible.

Guide is kind of unfinished but should give you a good idea of playing Maokai (I hope).
Get out there like a tree and leaf those other champions in the dust!