So, Tricky and WTFnoob are walking along in the forest when a giant enemy murloc appears.

Tricky uses Frostbolt on the murloc, doing 100 damage and gaining aggro, meaning it'll attempt to chomp his face off.
WTFnoob decides he doesn't want Tricky to die and hits the murloc for 70 damage, which will certainly annoy it but not cause it to switch targets. Seeing Tricky is low on hp, he then heals tricky for 1337 health. The murloc will then become very, very pissed off and go after WTFnoob. What a noob.

Shitty example aside you basically generate threat points for nearly anything combat related. Rogues just stealth bs out of everything but still. You can stop hitting something because you see your threat shooting up but poisons will keep ticking and keep doing more, etc. Threat is displayed at the top of the enemy's nameplate if it's a mob and sounds will play when you're about to/when you get aggro.

-A Shitty Tricky Tutorial