When referring to an online social network, a friend is a person who has connected their personal account to yours, allowing the two of you to share information with each other. In the case of Facebook, you must invite a person to become a friend before being able to see any of their personal information. If that person accepts, you become online friends.
If you are more familiar with Twitter, you can think of a Friend as a follower. However, when you invite someone to become a friend you will not be able to see that person's profile until they accept your friendship.