For example, on Facebook, you can select a user and click "Add as Friend" to send a friend request to that user. When the user receive your friend request, he or she may choose to accept or decline the invitation. If the user accepts your request, he or she will be added to your list of friends. Likewise, you will be added to that user's list of friends as the same time. MySpace includes a similar feature.
Once you become friends with a user, that person will be able to access your profile with the additional viewing rights. This means he or she may be able to view more or your profile and post comments on the "wall" of your profile page. Since non-friends may not be able to view any of your profile, it is common practice to accept most friend requests. Of course, this has led to a rather liberal definition of "friend," since many people have hundreds of online friends who they hardly know.
The word "friend" can also be used as a verb, which means adding a user as a friend. When you delete a friend, you "unfriend" that person.