Speculative fiction is an all-encompassing term which includes science fiction, alternative history (fiction), horror and fantasy.
The term is often used among writers and publishers who wish to break out of what is commonly called the "sci-fi ghetto". There is an unfortunate tendency, among both many publishers and a large segment of the reading public, to expect only science fiction stories from a person who has once written science fiction. Writers such as Harlan Ellison (an outspoken advocate of the term) have deliberately rejected identification as a science fiction writer for precisely such reasons; they don't reject the science fiction genre (in which Ellison, for example, still participates) but they do reject pigeonholing their work.
The abbreviation "sf" (usually spelled in lowercase, but occasionally uppercase) is often used to indicate either speculative fiction or what is traditionally known as "science fiction".
This term is coming into more frequent usage among younger fans who wish to break down the literary barriers between the horror, fantasy, and science fiction genres.
The term is also sometimes used without any implication of breaking down barriers or breaking out of the ghetto, simply as a convenient shorthand way to refer to multiple genres at once. A variation of this term is "Speculative Literature".
See also: Internet Speculative Fiction DataBase