Light novel is basically novel that's in series rather than a book that targets teens and young adults.
Even though it's in a series, normally each volume contains at least 300 pages.
The writing style for light novels is often very different from novels, it is simpler and more aimed for teenager.
Also there are anime like drawings of the characters and book cover drawings for each volume.
Light novels have become very popular in Japan, and the publishing companies are
always searching for new writers with annual contests, many of which earn the winner
a cash prize and publication of their novel. The Dengeki Novel Prize is the largest,
with over 2000 submissions annually.
Many light novels are being made into animes, for example "To aru majutsu no index",
"The Melancholy of Haruhi Suzumiya", and "kara no kyokai".
Also, many light novels are being translated into different languages and are published in some countries, such as Korea, USA and China.