Friends are just that. People you've gotten to know over an extended period of time and have decided to add them to the respective list. Once you've added them there, you'll be able to tell if their offline or online.
With Foes, you'll be able to ignore PMs and posts from someone that you've put there if they've been a jackass to you, aside from admins.
But other than that, it all depends on who you pick as your best bud or someone who's been a total jerk.