Superhuman can mean an "improved" human, for example, by genetic modification, or as what humans might evolve into, in the distant future. Occasionally, it could mean a "normal" human with unusual abilities, such as psychic abilities or exceptional proficiency at something.
Superhuman can also mean something that isn't human, but considered to be "superiour" to humans in some ways. A robot that easily passed the Turing test, and could do some things humans can't, would be considered superhuman. A very intelligent or strong alien could be considered superhuman.
See also: Superhero, Superman, The Incredible Hulk.