From Sega Retro
A savestate is a file generated by an emulator which contains all the data that was in the RAM, VRAM, Z80 RAM and all other parts of the console's memory at the moment of its creation. Savestates created by Gens or Genecyst have the extension .GS*, whereas savestates generated by KGen have the extension .KS*. These two types of savestate are not interchangeable (i.e. you can't load a KGen savestate with Gens).