The Differential Association Theory proposes that people learn criminal behavior by communicating and interacting with others who are already involved in criminal activities. People become attuned to criminal behavior by taking in more positive definitions of violations than negative definitions. Criminal activity begins to look desirable the more people associate with others engaged in it, and people learn through those associations, especially when they have more such interactions than they do with people who do not participate in such behavior.
Let's look at this by way of example. A young teenager feels like an outsider in his new school. He has little support at home and few friends, so he turns to a crowd in which he begins to find acceptance. Unfortunately, this crowd is involved in criminal activity. The youths brag about their acts, about the money they receive and the "conquests" they've experienced. Their new friend takes all of this in, laughing along with them because they expect him to. As time passes and the teen hears more and more of this talk, it becomes more normal to him, like it's not a big deal. He becomes desensitized to it, and his ideas of right and wrong begin to get blurry.
Since the teen has few other associations to show, the youths of his new friendship group slowly convince him that their criminal activities are fun and desirable, that they can benefit him by money and a sense of belonging. Pretty soon, the teen starts going along with his friends when they steal or fight or do drugs. He now associates criminal activity with something positive rather than negative, and this is the Differential Association Theory in action.