This strategy is one of the most common Zerg strategy. It is very effective against nearly all opponents.

-Build 9 drones, make an overlord.
-Make a pool, a creep colony, and and an extractor.
-Build more drones, mine gas, build a hydralisk den, make another hatchery, and make your starting hatchery a lair.
-Continue building more hatcheries and morphing Hydras.
-Upgrade hydra speed, attack range, and all 3 overlord upgrades. Also upgrade missile attacks and ground carapace.
-Continue building mass hyrdas. Don't forget to have spore colonies scattered around your base.
-Build 12-24 mutalisks.
-Build hydras until you can fill up 12 overlords (48 hydras), but keep some in your base.
-Look for a weak spot in the side of your opponents base. Go in with the mutalisks, and have your overlords drop every freakin' hydra into their base.
-Ta da!