A 3 Step Process :

We first need to look into Face Detection before Recognition. Face Detection involves finding the position of the faces in a given image. Once the faces are found they will be recognized by matching them to faces in a database using face templates, which we will discuss subsequently.

So face detection only gives us the answer to the question – “Where are the faces in the image?”. Such algorithms are also used in digital cameras along with smile detection algorithms. The resulting image looks somewhat like this:

So the output image has the faces marked with a green square outline. You must have noticed Facebook introduced Face Detection which they call their Auto-Tagging feature wherein, when you batch upload images, your friends are already tagged-

The method described here is called The Viola-Jones Method for Face Detection-or rather, for object detection. In general, the algorithm can be used to find any desired object-of-interest which in our case, happens to be face.

Note that there are many different algorithms to do the same. This is just one of them. Perhaps, a more popular, simpler or efficient one.