Replicants are Protoss microscopic robots capable of fusing together to copy enemy units. They were created by the Protoss in the aftermath of the first phase of the Second Great War. After analyzing the battles they had taken part in, replicants were created to better improve their war machine.
Replicants are floating ground units. Apart from massive units, they can replicate almost any unit type, including flying units and workers. When they do so, they also manage to copy upgrades (e.g. cloaking), even if they have not been researched. They can use their ability on any enemy unit within line of sight. The drawback however, is that Replicants are very expensive to create.
In the game series, the Replicant is a concept idea for a new class of Protoss ground support unit.
For more information see the Replicant on the parent site.