Facebook's latest feature has introduced an important new term, "subscribing," that describes a novel way to connect on the social networking site. Now, you must choose between "friending" someone and "subscribing" to them.
So what's the difference? Here's how Facebook explains it:
You should only add someone as a friend when you know them personally. If you don't know someone personally but want to hear what they have to say, subscribing is a good option. When you subscribe to someone, you'll only be able to see their public updates.
When you request to add a person as your "friend," she must review and approve the request. On the other hand, if someone has opted to include the "subscribe" button on her profile, you can subscribe to see her public updates without her having to review the request, or without being friends with her (Note that she'll be able to see that you subscribed to her). Facebook "friends" can see any profile information a user has made available to that particular group--it might include tagged photos and videos, comments, status updates, wall posts, and contact information, among other details--whereas subscribers can only view public updates.