Servo-skulls are drone-like devices fashioned from human skulls - often those of pious Imperial servants, low-ranking tech-adepts, or loyal adepts of the Adeptus Terra - and then given a rudimentary Machine spirit, support systems, and anti-gravity engine to allow them to hover and drift bodiless through the air. Presumably this chance to continue their work even after death is a great honour in the Imperium.
Servo-skulls are most strongly associated with the Adeptus Mechanicus, but are used throughout the organizations of the Imperium to perform specific tasks for which they are installed with the basic equipment required, usually a pair of simple manipulator pincers, a weapon, a pict recorder, surgical implants, stab-light, loudhailer, or various other devices for the many roles that exist.
The roles they fulfill include Librarium functionaries, medical assistants, lumen-bearers, proclaimers or scroll-carriers. Some are designed for military roles, and among these some are built with enhanced optical sensors to allow them to fulfill a scouting role.
They are also often used by Inquisitors as Familiars. In this case they are often mentally linked to the Inquisitor, allowing him to control them and see and hear through their senses. In general, Servo-skulls are useful for their ability to enter otherwise inaccessible or hazardous areas because of their small size and great maneuverability.
The Skull Probe is a military device utilized by Space Marine Chapters for reconnaissance. It is capable of moving by stealth, and quick flight faster than most units can move on foot. It is also capable of detecting other units concealed by stealth technology. Though unarmed, they can generate an electromagnetic pulse that temporarily neutralises an enemy structure, though the skull is destroyed in the process.