X-Corps was a paramilitary police force established by Banshee (Sean Cassidy), a former member of the X-Men. Its intention was to regulate good behavior by mutants rather than waiting for crisis to occur.
The group included a number of members of the disbanded Generation-X team (e.g. M, Jubilee and Husk), but also known terrorists/X-Men villains, such as Blob and Avalanche, who were kept under control by the imprisoned Mastermind.
The group shut down after Mystique (posing as new recruit, Surge) freed Mastermind and slit Banshee's throat - the source of his powers.
After disbanding, many former X-Corps members would find new homes for themselves in Xavier's X-Corporation.