Skills are your means of combat in Monster Girl Dreams. Each takes the form of a certain sexual act which you use against foes; in turn, many monster girls have unique skills which the player has no access to. Most skills cost Energy to use.
Skills can be obtained from Elena, Elly, Vivian and Amber in exchange for eros. Lillian also offers some skills once you meet the necessary requirements.
Monster skills can be found on their individual pages, see the Characters category for a comprehensive listing of all characters currently in MGD.