There are two types of images that can be formed: Real and virtual.

Real images

Real images are images that are formed at the same side as the either the reflected or transmitted rays. Rays actually passed through the points in the image and are being picked up by your eyes. As a result they can be focused on to a projection screen. By convention, the position of a real image is greater than zero.

Virtual images

Virtual images are those that appear at the same side as the incident rays. Rays did not actually pass through the image but you can virtually trace the rays back to the location of the image as if the rays originated from there. Any mirror or lens can produce a virtual image. Virtual images are described by a negative image location.