From my experience watching my brother play Zerg (He's in platinum and diamond leagues, so pretty good) there are several ways for zerg to win:
1. Early rush, doesn't work well often, but if you micro it well you can do enough economic damage to make the rest of the game a formality.
2. Harass: Nydus Worms/Mutas. Again the goal is to kill enough of his workers to throw him out economically. You don't always have to go for his main! In fact, by the time you do this, his main's mineral count will be far lower, and if you pull this off effectively and take his expansion, he'll be totally starved.
3. MACRO! Especially against a Terran player, it's easy for the Zerg to get map control, and while he's stuck inside his heavily defended base, you grab expansions and lots of them. With 4 bases, you can pump out oh so many units. A Terran ball may kick ass, but that won't matter when you have twice as many units as he does!
The Madman said:
They can indeed, and it is easier too. Nydus canal can only be built on creep so the zerg player had to sneak an overlord in there to lay down just enough before getting shot out of the sky.
Not true, you don't need to make creep, you just need sight. If he has sight, he can build a nydus worm ANYWHERE he wants to, and that's why it kicks ass. In fact he can build several if he needs to get his units through quicker, they're pretty cheap.