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).

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).