The Genesis Device was mentioned in Star Trek II: The Wrath of Khan. It was designed to alleviate sociological problems such as overpopulation and limited food supplies. Its development was completed by a team of scientists led by Carol Marcus and her son, David Marcus, in 2285 on the Spacelab Regula I in the Mutara sector.
The device initiated a process of rapid terraforming, by which previously uninhabitable planets could be turned into class M worlds ready for colonization. This was accomplished by launching the Genesis Device, a torpedo-shaped projectile, into a lifeless planet. Upon impact, the device caused a massive explosion, reducing the entire area to subatomic particles. A preprogrammed matrix then reassembled these subatomic particles into the desired configuration, creating an atmosphere and environment habitable for Humans within a matter of hours, regardless of the test area's original composition.
In Star Trek: Conquest, the Genesis Device is a special weapon that is launched only into enemy star systems. (It cannot be activated on star systems which you own.) Upon launch, the Genesis Device does 50% of the base hull damage, such that if a unit or structure was already damaged below 50%, it would be destroyed.