The image formed by any mirror is located either where the reflected light converges, or where the reflected light appears to diverge from.

A spherical mirror is simply a piece cut out of a reflective sphere. Its center of curvature, C, is the center of the sphere it was cut from. R, the mirror's radius of curvature is the radius of the sphere. The focal point F (the point where parallel rays are focused) is located half the distance from the mirror to the center of curvature. The focal length, f, is:

focal length of a spherical mirror : f = R / 2

This is actually an approximation. Parabolic mirrors are really the only mirrors that focus parallel rays to a single point, but as long as the rays don't get too far from the principal axis then the equation above applies for spherical mirrors.

If the mirror's inside surface is reflective, the mirror is concave; if the outside is reflective, it's a convex mirror. Concave mirrors can form either real or virtual images, depending on where the object is. A convex mirror can only form virtual images. A real image is an image that the light rays from the object actually pass through; a virtual image is formed because the light rays can be extended back to meet at the image position, but they don't actually go through the image position.