What are light novels?

A “light novel” usually refers to Japanese novels, typically written for YA audiences, which are supplemented by occasional illustrations. Typically, you have manga (all illustration - like a comic book), light novels (mostly words, but some illustrations), and novels (no illustrations, all text).

Many popular light novel series have been adapted in animation. Examples include: Sword Art Online, Log Horizon, A Certain Magical Index, Irregular at Magic High School, Shakugan no Shana, All You Need is Kill (adapted as a film in the US - “Edge of Tomorrow”) and Kaze no Stigma.

If you’re interested, a number of them have been translated by fans and posted online at various places. A google search will probably help you find them.
What are light novels?

Light Novels are basically the novels that include graphics and texts together to target the High School and Middle School students. ‘Light Novel’ derived the name from the Japanese term. The average length of the Light Novel is about 50000 words and they are usually illustrated with Anime and manga art style.