If played correctly, Hecarim is able to play many roles for the team (even a few roles at once). His large amount of sustain and short cooldown of rampage make him able to do tons of damage even building tanky. Hecarim can easily get out of many sticky situations and can also easily cause many sticky situations for his enemies.

Pros

-Amazing movement speed with devastating charge and no unit collision.
-Rampage combined with spirit of dread cause Hecarim to constantly deal high damage to many people while sustaining his health.
-Rampage deals high aoe damage with very short cooldown making Hecarim able to deal large amounts of damage without many attack damage items.

Cons

-Early game (before level 6) ganks are semi difficult.
-Onslaught of shadows is a weird ultimate and takes some time to get used to.
-because of high aoe damage and sheer size, Hecarim tends to be focused more.

Because Hecarim is so reliant on rampage, it is important to start at blue buff first, otherwise your mana will be completely depleted before one round around the jungle. After blue, you will work your way to the other side of the jungle, killing wolves, then wraiths. After wraiths it is a good idea to check if the middle lane needs a gank. then you will go to red, then finish off with golems. More important than the path you take though, is being aware of which lanes may need a gank. If you notice a lane is starting to be pushed you can skip some jungle farm and head towards that lane, if they continue to be pushed or you see an opportunity for a gank, then gank.

It really helps to start off with spirit of dread first, because if someone is leashing for you, some of the damage they and you do will be converted to health for you. Maxing rampage first is also really important because it gives you high aoe damage which will help you clear jungle camps with ease, and it gives you high damage when ganking. After rampage you will want to max spirit of dead for it's wonderful sustain, and then devastating chage. You don't want to get devastating charge first because although there is a nice cooldown reduction on it, the attack only damages one target and even at max level, it has a long cooldown. Rampage will give you much higher damage that is able to be sustained until you run out of mana.