A light novel (ライトノベル) is an evolutionary development from what used to be called pulp-fiction magazines. In modern times, Japan produces an enormous amount of fiction in the light novel format.
Light novels typically top out at around 40,000-50,000 words (though due to the distinctive differences between the Japanese language and English, it is difficult to directly compare word-counts between the two languages). Light Novels are generally published as A6-sized paperbacks with a dust-jacket, and frequently feature a cover (and often also some internal illustrations) by a manga artist.
These days it is more common for television (both anime and live-action) to be adapted from light novels than it is for them to be adapted from original manga. In the case of successful light novels, both television and manga adaptions are common.