On your profile widget, you can find "Follows" and "Followed by" beneath your avatar. On other people's profile widgets, you can find a "Follow" button. The following feature allows you to be notified via email when the user buys a bundle, uploads new media, or list a tradeable item.
To follow someone, click on their "Follow" button, and select the options you want. The options do exactly what they say, except "As It Happens", which aggregates every 15 minutes. You can send the notifications to the email we have on file for you, or choose a different email address to send them to.
You can check who is following you and who you are following by looking at the profile widget on your wall. Click on the number below "Follows" to see who you follow, and the number below "Followed by" to see who follows you. If you haven't followed someone in the "Followed by" list, you can click on their follow button to follow them. Otherwise, you can click on the "Following settings" button to change what you subscribe to and how often it is delivered. Click the "Unfollow" button to unfollow the user.
Please note people who you follow can leave comments on your wall, and affect your trade rating, so be careful about who you follow.