Light fantasy consists of stories that contain both magic and humor. This is sometimes contrasted with dark fantasy, although there is a very wide middle ground that would not be considered particularly light or dark. Light fantasy tends to be taken less seriously than other types fantasy (well, yeah, but you know what I mean). It is however, great fun.
Some of the more popular authors, and their best known series (or books) are:
Terry Pratchett -- Discworld series. (A hilarious world.)
Piers Anthony -- Xanth series. (A magical world of puns).
Robert Lynn Asprin -- Myth series. (Madcap adventures in multiple worlds).
John Morressy -- Kedrigern series. (A wizard who goes on silly quests).
Craig Shaw Gardner -- Ebenezum series. (A silly wizard).
Esther Friesner -- Majyk series, others.
Glen Cook -- Garrett series. (A gumshoe in a fantasy world).
Lionel Fenn -- Kent Montana series. (Movie monsters!)
John Dechancie -- Castle series (Craziness in a multi-dimensional castle).
Tom Holt -- Expecting Someone Taller, and others. (Silly, and he does good historical fantasy).
A lot of children's fantasy would be considered light fantasy--here's some that you (or a kid you know) might want to check out.
J. K. Rowling -- Harry Potter.
Roald Dahl -- all kinds of stuff.
Patricia C. Wrede -- Dealing with Dragons series.
Lynne Reid Banks -- all kinds of stuff.
Lewis Carroll -- Alice in Wonderland
Diana Wynne Jones -- Chrestomanci series, others.
James Thurber -- The 13 Clocks, etc.
Eva Ibbotson -- The Secret of Platform 13, etc.