Welcome to Mangahelpers Visual Novels Forum! Visual Novels are a type of video game popular in Japan, but rarely translated for North American or European release. They are typically text driven, with largely static, anime style art in the background, sometimes featuring more complicated animation between chapters. The reader is periodically confronted by "events", situations in which they have to choose a specific response, or action, before continuing. The choose the player makes determines what "branch" the story takes. Make the right choice, and raise "flags", leading to a good ending. The wrong choice can lead to a "bad ending".
There are visual novels for practically every genre. Romance are the most famous, but there are also many well known action, sci-fi, mystery, and drama games. Popular visual novels are often adapted into anime. Steins;Gate, Clannad, Fate/Stay Night, To Heart, and Amagami (among many others) were all based on popular visual novels.
Visual novels marketed towards women are referred to as Otome Games. Kiniro no Koruda, Alice in the Country of Hearts, and Uta no Prince-sama are examples of Otome Games.
Many visual novels contain some nudity and sexual content. This does not make them eroge. Eroge are porn, visual novels are not. Even many download sites mistake the two. Think of eroge as hentai, visual novels are manga. The format are the same, and manga can contain adult content without being considered hentai. When in doubt, use common sense.