Robot units are StarCraft units that consist entirely of mechanical components.
Colossus can be built by the Robotics Facility
The Zerg do not have robotic units, and there are relatively few types among the Terrans (eg spider mines). The Protoss have an entire structure (the Robotics Facility) devoted entirely to produce robotic units like Reavers or Colossi.
Robots are vulnerable to a variety of effects (such as a Ghost's Lockdown ability) and are immune to effects that affect organic units (such as a Science Vessel's Irradiate ability) and even some effects that affect mechanical units (such as a Queen's Spawn Broodling ability).