A savestate (sometimes known as a snapshot) is a file that emulators use to store and recall the entirety of a game's internal state at a particular point.
The bulk of the data in a savestate is the content of the game's RAM, but there is also other important state information, including the CPU registers, interrupts and open bus, memory bank controller state (e.g. ROM bank and SRAM bank loaded), PPU (picture processing unit) state, etc. The exact contents of a savestate may depend on the accuracy of specific emulators, since less accurate emulators may make approximations that remove the need for certain state information. As such, it is difficult to create a universal savestate format for different emulators.