Or maybe you really want all new sounds, but cannot afford an all new keyboard. Essentially it is everything a keyboard is, except for the keys. Time to get a little technical. When you strike a key on an electronic keyboard, the key hits a sensor. This sensor detects that the key was struck, how long it was held, and when it was released.
Some keyboards are velocity sensitive and can detect how hard a key was struck.
This information is fed into a computer that translates the key sensor's output into computer commands. The main brain of a keyboard then receives these commands and generates sounds as appropriate. Of course, this all happens very, very fast. Once you have generated the commands, any sound module can respond. It just so happens that the sound module that usually responds is the one inside the keyboard that you are playing.