Science fiction series revolve around some sort of new advance in science. This might be completely in the future featuring space ships, as in Star Trek, but it might also be set in the present day or past, such as the Jules Verne novels (and their movies).
Westerns usually involve stand offs between arch enemies or even friends, have a heavy soundtrack, and usually involve criminals which terrorize towns.
It's entirely possible to mix the two genres: Firefly is a very good example of a series which has science fiction and western elements.