When an object is placed in front of a concave mirror, light rays from the object fall on the mirror and get reflected. The reflected rays produce an image at a point where they intersect or appear to intersect. Formation of an image by mirrors is usually shown by constructing ray diagrams. To construct a ray diagram, we need at least two rays whose paths after reflection from the mirror are known. These rays must be chosen according to our convenience.
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Spherical mrrors are two types : concave spherical mirror and convex sph mirror.
The spherical mirrors have a radius of curvature R and a focal length f = R/2. They have a pole and a principal axis which is drawn to meet mirror at pole (center of surface) and perpendicular to mirror.

When an object AB of a height h is placed in front of the mirror on the principal axis, the light rays from the object parallel or at any angle to the axis, fall on the mirror and get reflected according to laws of reflection.  The image can be oobtained by taking rays :  from the ends of the object falling on the pole of the mirror and rays parallel to axis.  Image is formed where the reflected rays meet.
Image can be real image or a virtual image.
For concave mirror, If the object is placed at a distance more than the focus then a real image is formed. If object is too close , less than focal length, virtual image is formed. 
A convex mirror forms a virtual image, that is you can see only an image inside it like our mirror in bath room.  We cannot catch ir or project it onto a screen.
Images are also magnified or diminished in size depending on the object position.
For a very distant object, the real image is formed at the focal length for concave mirror. and a virtual image is formed by convex mirror at focus.

The distance at which image is formed is given by
   1/v + 1/u = 1/f      where u = distance of object from pole, f = focal length, v is distance of image from pole.