Master Yi is a champion of many possibilities. He can be played either AD or AP. When played AD, he can be glass canon, tanky damage dealer or offtank. I think its best to build him as tanky damage dealer. You still have absolutely amazing damage, while having decent health with good MR and Armor.

Pros
Great pusher/tower/farmer destroyer
Pretty easy to learn overall
Great escape/chase mechanism
CD resets/halves when you get a kill/assist(if Highlander active at that moment)

Cons
Vulnerable to disables like blind, stun,suppress...
Low HP poll until you get Frozen Mallet(or other HP item)
Not so great in team fights

Flash

Teleports your champion toward your cursor's location.

One of best spells IMO. Its useful both in offensive and defensive tactics. You have been hopelessly ganked? Flash away. Enemies chasing you? Flash over the wall. Enemies got away with 50Hp? Flash to them and deal 1 more attack. I love this spell, even after the nerf in November 2011.



Smite

Deals 445-870 true damage (depending on champion level) to target enemy minion or pet.

This is obviously a must pick for junglers. Not only it helps you jungle faster than you could with any other spell, it also grants you amazing buff controls. I can't stress how important smite is, I personally have stolen Baron more than 10 times with Smite.



Ignite

Ignites target enemy unit, dealing 70-410 true damage (depending on champion level) over 5 seconds and reduces healing effects on the target for the duration.

This is another great spell for Master Yi. It deals a lot of damage and is great to finish off champions with this extra damage. Ignite is also useful because of 50% Healing reduction. With Ignite you can shut down enemies like Swain(**** on their healing capabilities) easily.



Exhausth

Exhausts target champion, reducing their Movement Speed by 40%, attack damage dealt by 70%, and ability and item damage dealt by 35% for 2.5 seconds.

This is great spell and like Flash and Ignite, works miracles both in offensive and defensive tactics. When chasing, this spell reduces enemies Movement Speed by 40% for 2.5 seconds, which makes it a great offensive tool. When using it defensively, this tool protects you by nerfing attacker's basic attack damage by 70% and item/ability damage by 35% for 2.5 seconds.



Cleanse

Removes all disables and summoner spell debuffs affecting your champion and lowers the duration of incoming disables by 65% for 3 seconds.

While I rarely use it, it can be still viable on Master Yi. This champion is extremely vulnerable to CC disables and if you do not intend on buying Quicksilver Sash, this spell is kind of must take. Since November 2011 it also removes Ignite, which is great.



Teleport

After casting for 4 seconds, teleports your champion to target allied minion, turret, or ward.

This spell is viable if your team has no pushers and they rely on you for pushing. When enemies are attacking your tower in group, just teleport to pushed lane and destroy towers quick. Hidden pushing is something Yi excels at.


Masteries

In masteries I take 21/9/0. Master Yi benefits from 21 points in offense so much, that going any other way is just absurd. You get all that extra damage and survivability, you can't deny that. I use 21 in Offense both for laning and jungling.
21 OFFENSE(JUNGLE AND LANE)


Tier 1( Butcher// Brute Force// Summoner's Wrath)
First tier gives doesn't provide you much, so you should take such masteries from which you benefit most. I take Summoner's Wrath for improved Exhaust, Butcher for easier jungling and Brute Force so I can progress to second tier.