Visual novel
A visual novel is a Japanese adventure game that only remains a game in the loosest sense of the word. As the name might suggest, they resemble mixed-media novels or stage plays more then anything else. Minimal player intervention is required, only clicking to keep the text, graphics and sound moving (some VNs offer 'play' or 'fast-forward' toggles that make even this unnecessary) and an occasional decision that affects the final outcome of the story.
Visual novels have been compared aptly to Choose Your Own Adventure novels. Most, however, strive for a higher level of plot & character depth then the aforementioned series of interactive children's books. Many fans of visual novels find them a welcome oasis in the often-desolate landscape of traditional video game storytelling.
Visual novels were a subset of ren'ai games in the past, and almost all contain hentai, which English-speaking fans tend to regard as a sort of unpleasant and unnecessary cliche (think of the 'obligatory sex scene' in Hollywood action films.) However, many recent ones have placed both titillation and romantic interaction decidedly low on their agendas, and they can be regarded as a distinct genre from ren'ai and H games.
Famous visual novels
Many of these also have associated anime series.