What is characterization?

In literature, characterization simply refers to the process by which an author constructs a fictional character.
Depending on the work and on the individual character concerned, this can be done in a number of different ways. For instance, a character can be given a specific manner of speaking which sets them apart from the other characters. They can also be characterized according to their style of dress, how they walk, and their physical movements in general.
All of these characteristics and more are used to present a character to the reader, allowing us to make a judgment as to what kind of person they are. In some cases, characterization will be more complex, more subtle, in keeping with the nature of the individual concerned. In other cases, characterization will be much broader, especially with secondary characters designed by the author to add color or humor to the narrative.