A virtual image (as opposed to a real image) is produced by an optical system (a combination of lenses and/or mirrors) when light rays from a source do not cross to form an image. Instead they can be 'traced back' to a point behind the lens or mirror. Virtual images can be seen directly without using a screen for projection.
The most common virtual images are those produced by a flat or plane mirror. As shown in the diagram below, the virtual image appears the same distance behind the plane mirror as the object is in front of the mirror.