| Mayzie's writeup is largely accurate, but is missing a few crucial points. It is important to note that some models of replicants are superior to human beings both mentally and physically. Replicants were given short lifespans not only to prevent them from developing a "sense of humanity" but also to prevent them from staging a successful revolution against their human masters (coincidentally, this probably allows the Tyrell Corporation to sell more replicants!). Replicants are outlawed entirely on Earth due to the Earth government's (justified) fears of their capabilities.
Interestingly, Bladerunner director Ridley Scott has stated that the main character, Deckard, is indeed a replicant.
Replicants are frequent plot devices in science fiction films and written work. They do not always appear by this name, however (cf. androids, Buma, boomers). | 
