Orcs and Humans never liked each other. They started to fight, each race gathered up other races to create the horde and alliance. They fight alot. Other enemies come along and the pair join up and fight the common enemy, but they're still fighting against each other. Thrall is cool. The other !#!gy alliance leader is not. 

No but seriously, if you want a basic outline to the WoW story line go to youtube and type in "Warcraft Lore" and there are about 10 videos in which basic outlines to the story are explained.