How do mirrors form images?

Recall that light reflects from mirrors, according to the law of reflection.  However, it the mirrors is curved, light still obeys this rule - it just looks a bit different.  You have to visualize the curved mirror as a series of little flat mirrors.

A convex mirror (top) acts just like a concave lens - only virtual images are formed.  Think of convenience store mirrors.

 A concave mirror (bottom) acts just like a convex lens.  Think of makeup/shaving mirrors.