When most people hear th­e term social network, they automatically think of online social networks. That's because online social networks, also known as social-networking sites, have exploded recently in popularity. Sites like MySpace, Facebook and LinkedIn account for seven of the top 20 most visited Web sites in the world. For many users, especially the fully wired Net Generation, online social networks are not only a way to keep in touch, but a way of life.

Several features of online social networks are common to each of the more than 300 social networking sites currently in existence. The most basic feature is the ability to create and share a personal profile. This profile page typically includes a photo, some basic personal information (name, age, sex, location) and extra space for listing your favorite bands, books, TV shows, movies, hobbies and Web sites.

Most social networks on the Internet also let you post photos, music, videos and personal blogs on your profile page. But the most important feature of online social networks is the ability to find and make friends with other site members. These friends also appear as links on your profile page so visitors can easily browse your online friend network.

Each online social network has different rules and methods for searching out and contacting potential friends. MySpace is the most open. On MySpace, you're allowed to search for and contact people across the entire network, whether they're distant members of your social network or complete strangers. However, you'll only gain access to their full profile information if they agree to become your friend and join your network.

Facebook, which began as a college social network application, is much more exclusive and group-oriented. On Facebook, you can only search for people that are in one of your established "networks." Those networks could include the company you work for, the college you attended, or even your high school. But you can also join several of the thousands of smaller networks or "groups" that have been created by Facebook users, some based on real-life organizations and some that exist only in the minds of their founders.

LinkedIn, the most popular online social network for business professionals, allows you to search each and every site member, but you can only access the full profiles and contact information of your established contacts -- the people who have accepted an invitation to join your network (or have invited you to join theirs). You can, however, be introduced through your contacts to people who are two or three degrees away from you on the larger LinkedIn network. Or you can pay extra to contact any user directly through a service called InMail.