Base
These structures are for utility purpose.
Defensive
These structures kill Terrans when they get close.
Spawner
The Zergs source of highly replenishable army. These structures periodically spawn a unit and send it towards the Terran bases.
Each Spawner has a limit to the number of active units it can create at a time (upgradeable). At the start of the game only Tier 1 Spawners are available, i.e. Slowling, Creeper and Strikeling spawners. By upgrading the armor level at the Hive, the Zerg gets access to higher tiered Spawner. The higher the tier of the Spawner, the more Armor and Base Damage its units have, but also more supply each spawner costs as well as lower maximum unit counts.
Some spawners have special upgrades to make its spawned units more powerful.
Spawner pathing can be manually set (like a rally point) to change the route which units take after spawning. Can be chained with shift + right click.
Advanced Spawners
Advanced Spawners can be hatched by Infestors only. They spawn at a slower pace than normal spawners but their breeds are strong and useful. Some of these special units have either special abilities or a remarkable power, which balances their great cost.
Other