Pros:

Tanky as hell.
Fast as hell.
DPS like a carry.
Destroys towers.
Destroys Dragon and Baron (can 2-man baron as early as level 10)
Strong shield and sustain in lane
Fastest jungler in the game.
Great tower diver with turtle shield.
Solid counter-jungler.
Wriggles = FREE WARDS.
Extremely Versatile (can build AD, magic damage with Phoenix/AS, or pure Tank)

CONS:

Has to push lane to sustain mana. (if you're not jungling)
Low CS (minion kill score) unless you play Phoenix Udyr (build 3)
VERY expensive item builds.
Has to run to initiate...is countered by an alert player using Flash.
Lacks Pig Stance (so no true ManBearPig status)
Turtle nerfed to the point where a blue pot early isn't a bad idea.

Your passive. Makes you tankier and gives you AS. Keep this in mind and stance-dance as often as you can in tough fights (enemy champs, Dragon, Baron)



Your bread and butter. Gives you a ton of natural attack speed, so you don't have to get it from gear.



This skill is the difference between good and bad Udyr's. THe shield created by this skill can absorb a turret shot when you're diving an enemy or sevaral later on when you tank towers so your teammates can kill them without waiting for minions. It can be a thousand more HP in a 1v1 fight if you dance properly...not to mention the sustain it gives you, saving trips back to base.



Your run and stun skill. The closest thing you have to an initiation skill. Can be used on one target every 6 seconds, but if you go from person to person in a teamfight, can stun EACH of them once...can make a big difference in the fight just with this skill.



This skill farms creep waves very well. That's pretty much all it does, besides looking cool. If you build around it, (see build 3) then you use it more often and become more of a farmer than a champ killer. Personally, I avoid it altogether and never miss it.

Its absolutely this simple. to catch them and land initial stun, to DPS them, just to pop up the shield for added survivability. Repeat if necessary.

Jungling is simple. GET A PULL ON BLUE GOLEM. Put your ignite on it as soon as the pulling champ leaves. (Don't flame me for this, it makes your already fast jungle even faster.) Smite once it gets to 550 health. Clean up wolf camp and move to mini-golems, unless your smite is almost up again. If Smite is ready (or within 20 seconds of being ready, start on Red Golem. Once Red buff is secure, finish mini-golems and Wraiths and move to gank whichever lane the enemy is pushing the hardest.

PING YOUR GANK FIRST!!! Your teammates are ******ed. They need all the help they can get, ESPECIALLY in lower ELO.

Once you have Wriggles, put up a ward (always use your free ward) in the bush below mid and then begin thinking about getting Dragon. The enemy almost always will ward it, so be aware of this and only make a move for dragon after a successful gank, and PING THE BASTARD OUT OF IT so your team will come help you. The faster it drops, the better, as you are a sitting duck if you get ganked with that lizard DPS'ing you.

At this point, you are DONE jungling. Thats right, now you're a regular champ like everybody else. Push lanes and only farm jungle when you don't have a safe lane to push. HELP YOUR TEAM!!! I can't stand junglers who are getting a buff instead of joining a teamfight that they are the difference between winning and losing. Trust me, that red buff is NOT more valuable than a turret or lost teamfight.

Your core build is Trinity Force + Atmas + Warmogs. This is the core of Tanky DPS, which is a major part of the current metagame. These items are crucial. On the Tiger stance builds, Wriggles is also mandatory, as it pays for itself in free wards and really picks up your farming/jungling ability, something that Tiger stance lacks.

Ninja Tabi synergize very well with Udyr, and should only be substitued out for Merc Treads when the enemy team is LOADED with AP or CC. Even then, more often than not, you're better off with Ninja Tabi.

Endgame, you can sell your Wriggles and buy a situational item, I recommend Madreds or Infinity Edge for damage/synergy with your other items.

BUY WARDS!!! The one you get from Wriggles is nice, but your team can always use more. This goes double if you're playing Phoenix Udyr as you don't have high Bear stance for escaping, you need that advance warning time to save you from ganks.