A visual novel is an interactive fictional work, similar to 'Choose your own Adventure' books.
Visual Novels put the player in the shoes of a Protagonist, and you follow his or her life throughout the game. The player gets to make decisions based on what he/she decides is their preferred choice of action. For example, some Visual Novels are dating sims, so your choices can change which boy/girl you turn up with, and how the relationship works out.
Visual Novels are mainly based for PC, though popular ones are sometimes ported to consoles like Dreamcast and Playstation 2. Visual Novels rarely make it to the West, though popular titles like the Ace Attorney Series sometimes get through.