Exocomps were small machines that could easily fit inside a 1 meter cube. They had anti-gravity devices for locomotion and a small replicator mechanism that could generate an assortment of tools. Their purpose was to fly through the ductwork (Jefferies tubes) of a small space station to perform maintenance and repairs. They were built with enough artificial intelligence to diagnose problems and determine how to correct them, but they developed sapience as an unforeseen side effect.
Discovery of their sentience led to exocomps not being allowed to be used on a general basis in the Federation. This is odd, because they would later have no problem sending possibly sentient holograms, EMH mk1 systems, to do menial work in mines and plasma conduits. This ruling seems to be linked to the ruling involving Data in TNG "The Measure of a Man." Why it applies to these utility robots and not holograms is hard to understand.