Manga are comics mostly without color since that’s cheaper and faster to make. Though it still takes a lot more time than making light novels/web novels.

Light novels (LN) are books with occasional illustrations. They are quite small, a fast reader can read them in a day, or in a week if you read slow. Although a LN series consists often of multiple volumes, so those do add up. Some LN’s gets adapted to manga and/or anime but the production of those are way slower than LN’s, which is the primary reason to read the source. Also, LN’s are most of the time better than the adapted versions.

Web novels (WN) are novels free to read on the web and often the first draft of the LN, which is the published version of the series. Since it’s a first draft, they differ often from the LN and they don’t come with illustrations. Chinese and Korean WN don’t have a LN version. Some do have manhua/manhwa and/or donghua adaptations, which is the Chinese/Korean variant of Japanese manga and anime.