Perhaps the easiest way to understand social networking is to think of it like high school. You had friends in school and you knew quite a few people even if you weren't friends with all of them, but it's likely that you didn't know everyone.
If you ever moved to a new school, or if you can imagine moving to a new school, you start out with no friends. After attending classes, you start meeting people, and as you meet them, you begin associating with those that have similar interests.
Getting started with social networking is much the same as starting a new school. At first, you don't have any friends, but as you join groups and begin meeting new people, you build a friend list of those with similar interests.
Attending a job fair to learn more about businesses in your area, is also a form of social networking. You may have heard that social networking is important in finding a job. This is true in that knowing people (social) and communicating with them (networking) can help you land a job possibly easier than someone who isn't going down that route.
In the context of the internet, this is what social networking is, except online.
Social networking is based on a certain structure that allows people to both express their individuality and meet people with similar interests. Below are a few common components found in most social networking websites.