How does Chen "work"?

You're not meant to go late with Chen. If you're picking Chen you want early ganks (preferably kills) and then transition into pushes. Your 3 main ganking creeps are the Centaur with the stun, Troll with the snare, and Hellbear with the slow. If you cant gank then farming is fine, but with a hero like Chen you really want ganks to be effective. Eventually you'll start pushing towers. For that purpose, the Wildwing and big crit wolf are both great. Wildwing gives armor to all your allies, and the big crit wolf gives 30% dmg to all allies.

A trick for (mostly Dire) jungle. Stack a camp multiple times, like 3-5, pray to find a Wildwing creep (the big bird with tornado and armor aura). Cast tornado on the stacked camp for big XP and gold burst. You should be stacking every time if possible because Chen can clear stacks fast even without the tornado.

The abilities.

Centaur Conqueror

War Stomp (2 second aoe stun, also deals damage)
Swiftness Aura (+15 attack speed, like a free Drum of Endurance)

Dark Troll Summoner

Ensare (decent cast range, 1.5 seconds, disrupts channeling)
Raise Dead (can summon 2 low hp skeletons with decent dps for jungling)

Satyr Tormentor

Shockwave (spammable damage that hits units in a line, real easy to break salves and clarities early game)
Unholy Aura (+4 hp regen, like a free Mekansm aura)

Satyr Banisher

Purge (can remove debuffs from allies, removes buffs from enemies including runes, also deals 400 damage to summons and illusions)

Alpha Wolf

Packleader's Aura (+30% damage to all allies around it, just try and tell me it does "nothing" I dare you)

Hellbear Smasher

Thunder Clap (3 second aoe slow, slow is 25%, also deals damage)

Harpy Stormcrafter

Chain Lightning (free Mjollnir zaps)

Wildwing Ripper

Tornado (summons an invulnerable tornado, free harass, you can deal unavoidable damage with this)
Toughness Aura (+3 armor in an aoe, pretty strong, especially early game)

Ogre Frostmage

Ice Armor (basically a lot like Lich's armor spell, gives +8 armor and slows enemy attackers)

Ghost

Frost Attack (20% movement slow, like having a drow on your team sort of)

Kobold Foreman

Speed Aura (+12% move speed, similar to having a mini spirit breaker, that's actually A LOT of bonus speed)

All this information is also useful if you're playing Doom. He can make excellent use of neutral creep abilities too.