The Zerg have buildings that must build on the "creep," a living carpet that gives supplies to all the zerg buildings whichare actually huge living masses. The zerg unit itself is generally very weak compared to the Protoss and the Terran. But all their stuff is very cheap and you can build 3 things at the same time from one building. But all their units build at one building so its usually wise to build many of them. The two zerg units i use the most are hydralisks (pictured above) and guardians. Hydralisks shoot spines at air and ground units, while the guardian is a flying crab looking thing that shoots only at ground with a very long range.