Workers

Workers (AKA peons) are the units that mine minerals and Vespene gas and construct buildings. They are the Drone (Zerg), SCV (Terran), and Probe (Protoss). All three types of workers cost 50 minerals to produce.

Importance
Workers are usually produced in large number to assist in developing a player's economy since nothing can be made without resources. Most players will use the 50 minerals they start with to train a worker and continue building workers for the first five or so minutes of the game. Especially for terran and protoss, most players will not stop worker production at at least one Command Center/Nexus until the late game (zerg players need to be more aggressive and expand more so have less workers per expansion). Other than their HP and method of constructing buildings, these units are identical among all races. Workers are quite vulnerable without support.

Construction
Each race's worker unit has a different way of constructing that particular race's buildings. SCVs construct them by starting out with mere rubble, and eventually, the building gains more and more shape, until the point where it is finally finished. The constructing SCV cannot be controlled by the player during construction; he or she must "halt" work on the building or "destroy" the building. Probes warp buildings, which they create using a warp gate, and eventually, the building arrives at it's destination. After Probes create the warp gate, they can be assigned to another task, due to that the building will finish up itself. Drones mutate into the buildings, which means the drone will implant itself on creep-covered ground, and will start constructing the building. When the building finishes, not only will you lose the ore/gas required for the building, but you'll lose that drone too, so it's quite handy to rebuild another drone to maintain the Zerg economy.