Sci-Fi has the following meanings:
1. For the majority of the population, Sci-Fi is simply an abbreviation for Science Fiction. It was coined for this use by Forrest J. Ackerman.
2. However, among many in science fiction fandom (and a number of SF writers as well), Sci-Fi is (less than it once was) used as a derogatory term to indicate low-quality or cliche-ridden or worn-out examples of the genre. Hence the early distaste in the fan community for the Sci-Fi Channel — the perception was that no one who was really a science fiction fan would have given the channel that name, so it was likely being run by clueless suits blindly trying to exploit a niche, even though Isaac Asimov and Gene Roddenberry were part of the channel's advisory board. For many, the jury is still out, although the channel has had some remarkable successes (and numerous failures).
The Sci-Fi channel announced in March 2009 that they intend to change their name to Syfy. Unfortunate Implications ensue, unless the channel will really air more shows about sexually transmitted disease.
When used in this sense, Sci-Fi may be pronounced "sigh-fie," or may be pronounced "skiffy" and occasionally accompanied by a remark about the stuff sticking to the roof of one's mouth. (Explanation for non-Americans: there's a popular brand of peanut butter in the States called "Skippy." The advertising slogan "If it tastes like peanuts ... " has been parodied with the line, "If it tastes like cardboard, it's gotta be Skiffy!") "Skiffy" may be used as a derogatory term and when criticizing bad Sci-Fi channel decisions; even those who pronounce it "sigh-fie" often use the term "skiffy" in this context. It essentially dismisses an effort as being The Theme Park Version of "real" science-fiction.
The term (and spelling) "skiffy" has also been adopted by those who feel the whole "Sci-Fi" vs. "Science Fiction" vs. "Speculative Fiction" debate is silly, and many SF authors proudly proclaim themselves to be "Skiffy Writers". Mike Resnick published a collection of stories about SF writers in Alternate History called Alternate Skiffy.
Occasionally, some people still refer to "stf", an abbreviation of "scientifiction". (Pronounced "stef".) This may be because it inflects easily. ("Stfnal", "stfnic".)
For further details, see: http://www.wikipedia.org/wiki/Sci-fi
