Science fiction (Sci-Fi for short) is a genre of literature, film, theater, and other forms of expression that are based on speculations about current or future science or technology. It is notably difficult to define what is and isn't Sci-Fi as such stories can be set in the future, present or past, and on alien worlds, alternate forms of Earth or even Earth as it is or was historically. Even defining Sci-Fi as being a medium that contains at its core scientific or technological concepts that are used to either define, drive or resolve the story is inaccurate enough that, as a description, it would include such works as Bones, CSI, etc; which are notably not Science Ficiton works.
Science Fiction is now considered to be a separate genre to Fantasy, although this has not always been the case, with the genre being known as 'Science Fiction and Fantasy' comparitively recently, and in the past the two genres being conflated into the single genre of 'Science Fiction'.
Notable Science Fiction Series
The Foundation series - Isaac Asimov
The Star Wars novels (currently numbering in excess of 130) - Various Authors
The Ringworld series - Larry Niven
The Gap series - Stephen Donaldson
The Lensman series - E.E. "Doc" Smith
The Skylark series - E.E. "Doc" Smith
2001: A Space Odyssey - Arthur C. Clarke