Feature wallpaper (a.k.a. "accent wallpaper") is a wallpaper whose design differs from the design of the other walls in the room. The accent wallpaper's color can simply be a different shade of the color of the other walls, or have a completely different design in terms of the color, patterns and texture.
The surface of the feature wall can be wallpaper, paint, brick, marble etc. As their name implies, accent walls are typically intended to break up the pattern created by the other walls, especially when the walls are painted a single flat tone. However, a room may be wallpapered and have an accent wall that is textured and undecorated, as it would serve the same purpose.