Face recognition is the process of putting a label to a known face. Just like humans learn to recognize their family, friends and celebrities just by seeing their face, there are many techniques for a computer to learn to recognize a known face. These generally involve four main steps:
Face detection: It is the process of locating a face region in an image (a large rectangle near the center of the following screenshot). This step does not care who the person is, just that it is a human face.
Collect and learn faces: It is the process of saving many preprocessed faces (for each person that should be recognized), and then learning how to recognize them.