You may have noticed on your profile page that one of the numbers listed there is your number of followers:
Who are your followers?
Most likely your followers are your friends. When you add a friend, you are automatically following each other. However, someone who is not your friend may also follow you. In this case you won't know who is following you if they aren't on your friends list.
Your followers will get an alert when you post new content (such as an article, blog, or new forum topic), and when you come online.
Privacy and your followers
If you don't want your followers to see your blog, you can set it to 'friends only' in the privacy section of your settings.