Mechanical units are composed of mechanical components, though some possess biological components in the form of a driver. Generally, mechanical units are terran and protoss, with some infested zerg units also being mechanical.
Due to their biological components, mechanical units are vulnerable to some special abilities, such as a queen's Spawn Broodling attacks. However, they are immune to some special abilities which can affect organic units, such as the science vessel's Irradiate ability.
A "subtype" of mechanical units exist in StarCraft - robotic. This category includes reavers, shuttles, observers, spider mines and interceptors. Robotic units are immune to certain spells mechanical units aren't - for example, they can not be the victim of spawn broodling - but otherwise, they are very similar.
Wings of LibertyEdit
Science vessels can repair nearby mechanical units.
Heart of the SwarmEdit
In multiplayer SCVs can repair mechanical terran and protoss units.