When you "friend" a user on a social networking website, you add that person to your list of online friends. When you remove a person from your friend list, you "unfriend" that user.
Unfriending may be done for many reasons. For example, if an online friend is contacting you too often or writing on your wall too much, you may want to remove that user from your list of friends. You may also wish to unfriend a user if a relationship with that person has ended. When you unfriend someone, that person no longer has "friend" access to your profile page. This means he or she may no longer be able to view your profile or post comments on your wall. You can customize these options for friends and non-friends in Facebook using the "Privacy Settings" link.
Some situations may require a "mass unfriending," which involves removing many people from your friend list at one time. Many MySpace and Facebook users have hundreds or even thousands of friends, though most of these people are not close friends in real life. Since users with "friend" status can view your personal profile data, it might make sense to unfriend users that you don't know.