This build revolves around on hit effect items. This is when an enemy is hit with your basic attack, that the items passive gets set off. Now having majority of your items have passives like this, can kill an enemy champion very quickly.

Items like Witts End and Malady do damage with each hit, which are great with Teemo's high attack speed from this build. Madreds Razor does 4% of the targets health to them in magic damage, which adds up very quickly with 2.5 attack speed. Good usage of all these on hit effects can drop an oppenent rapidly.

Teemo's E "Toxic Shot" deals posion damage with each hit. This damage along with Witts End and Malady, add up to roughly 200 additional damage for each hit when toxic shot is maxed. That plus your fairly high attack damage, and quick attack speed, will dominate in a team fight.

Malady is a must on Teemo, but going completely over board with the on hit effects and attack speed is not only effective, but a fun, different way to play any type of champion.

The summoner spells used for this build are quite common. Flash and Ignite work perfectly so you can go into a fight, ignite them (along with your toxic shot and items passives) and secure kills easily.

People think Teemo's strength is in his early game. It is, in a way, but he is also very vulnerable. Play early game with caution and do not overextend unless you are sure that the chances of you getting ganked are low. Get an early phage after wriggles so you are somewhat tanky and you can control melee AD's pretty well once you get procs.

Here is where Teemo starts to shine. Shrooms are doing a !@#$ton of damage and the enemy lanes are barely getting their builds going. Wait until around 10-11 and ask for a red buff. Save 3 shrooms, Push your lane and force a dragon. Drop shrooms around dragon in case they come to contest it, and you will surely win this fight unless your team is just newbing it up.

(Edit after Teemo nerf - Shrooms still do solid damage, but just play more carefully as it is harder to bait kills with nerfed shroom damage. The increased CD isn't too noticable, the playstyle basically remains the same.)

Teemo's weakness is in late game. At this point you need to use Teemo's strong map control and possibly initiate a dual push. Teemo can push a lane by himself while the rest of your team is at another lane. If all go for you, back off and kite them through shrooms while your team gets free towers. If 1 goes for you, you should be able to 1v1 him, kill him, and take free towers while the rest of your team does a teamfight in another lane.

Teemo has a hidden passive which is Global Taunt. This means that in every teamfight Teemo will be the first to get focused by tanks and assassins. Your job is to kite everybody who focuses you through shrooms, survive, and possibly pull a kill as you do this. You should not worry about blinding enemy carries, or dropping shrooms in the middle of all the chaos as a smart team will seize any opportunity to land a stun on you and drop you like its hot.

As soon as you get a madreds, your ready to kill baron. By this time you should have a few shrooms to barons left and right choke points, and hopefully your roamer has oracled and has cleared wards around the area. In high elo play, if your team has disappeared off the radar for a length of time the opposing team is going to think that you are up to something. Baron many times isnt worth the risk, and be careful that you arent going to lose your advantage if you end up getting wiped and giving the other team a free baron.

If the enemy stacks MR, your other carry will need to capitalize because your damage will be sub par. Bot lane needs a farmable tank and roam, the tank should have good pushability in case the other team is gankhappy and roams 4 top, he can capitalize on that and push a tower or 2 while you hold the tower. Always good to have a jungler and roam, roam janna is good because she can come to your lane and make you even more of a juggernaut with shields etc.

In lane : Place shrooms in areas that will not be hit by creeps. Protect shrooms from creeps. The closer shrooms are to the main creep path, the higher the chance enemy champions will hit them giving you some extra harass in lane, which can lead to kills.

Outside of lane : Place shrooms so that you have them stretched to cover the river. If you are pushing hard, help mid with a shroom in the brush towards you.

Choke points : Any choke point with brush is good because even if the enemy oracles, they cannot see the shroom until they run over it. Choke points also give map control, because if you have a choke point with a shroom you have eliminated that much of the map that the enemy could be. Kinda hard to explain but yeah you get the point.