StarCraft/Zerg strategies

Introduction
Zerg has one major advantage on the ground: Almost all of its ground units burrow. This advantage can be manipulated very well. Burrow must be researched by the Hatchery/Lair/Hive, and only the Lurker can burrow without the ability being researched.

Finding Expansions
Zerglings very easy to make, fast, small, and can burrow. This makes them perfect for finding expansions. When Burrowed at a mineral depot, they can be an early warning sign if your opponent decides to expo. If the Zerglings are Burrowed right, the opponent will not know a Zergling is there, but will build their expo in a more inconvient spot (buildings must be built a specific distance from the resources, and can not be built on top of burrowed units). However, Terran Command Centers can lift off and reposition themselves to a more favorable position, so this does not work against Terran.

Ambushing
The key to ambushing is to make the right combination of units: Hydralisks, Zerglings, and a Defiler is a choice combination. The Zerglings engage in melee while the Hydralisks Engage from afar. If things get too messy, the Defiler can cast Dark Swarm. Using Dark Swarm will render your Zerglings virtually invincible, since only melee units can hurt them. This is especially useful against Terran, where the only combat melee unit is the Firebat, not normally used en masse.

Defilers
Burrow is very useful for a Defiler. This will help them survive against combat units, and also lets them regenerate hit points.

Lurkers
The lurker is the only unit that burrows without researching it. Burrowed lurkers can drastically help a Zerg player. They can decimate M&Ms and other infantry units.