Friends are members that you follow that also follow you back. You can view who these members are by clicking the Friends tab on your profile page. Once there, you'll see a micro-profile view of your friends, as well as how many friends of those are currently online and offline on the forum. Lastly on the page you'll see recommendations of other members that you might be interested in following on the forum.

You can also view the friends of other members as well, simply by visiting their profile and then clicking the Friends tab on the page. Once there, you'll be able to see a list of friends that the member has on the forum, along with the same online / offline tally letting you know how many of them are currently active on the forum.