With an online social network, a friend is an individual who has connected their account to yours, allowing the two of you to share information. With Facebook, you must invite a person to become a friend before being able to see any of their private information or pictures. If that person accepts, you become online friends.
If the time comes when you no longer want to be someone's friend, you can unfriend any of your friends. When you unfriend someone, they are not notified, but they may notice you're no longer listed as a friend if they look at their friends list.
Social networks also allow you to either hide or mute other users. If you don't like seeing what your friend is posting, you can hide or mute them to stop seeing their posts but remain 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 cannot see that person's profile until they accept your friendship.