A friendship is more so just someone to hang out with. You would probably share some sort of interests. Generally you spend less time with a friend, than you would with someone in a relationship. "Just friends" usually doesn't involve any sexual activities. And MOST of the time, you don't think of eachother has anything other than a friend. Basically, a friendship would pretty much be like any other friendship you have, male or female.
A relationship on the other hand, is much more intimate. Physically and emotionally. You would probably spend more time with this person, than someone who is just a friend. Your feelings would be different towards eachother, compared to just a friend.
Those are the more obvious differences...
As far as you guys being just friends after having been intimate together... It's possible. If it seems like she's ignoring you and doesn't want to talk to you, maybe your guys' experiences together made her uncomfortable right now. For whatever reason, perhaps she's scared that ruined the friendship, or who knows?! If I were you, I would just back off for a bit. I mean, I don't know how often you try to talk to her, but personally, I would give it a bit before I try being her friend again. Especially if you guys had recently been intimate together. She may still feel uncomfortable about it, and might be worried you want something more. Give it some time and let her contact you.
Good luck
laura