"She's a friend of yours..."

She is? I do not recognise the person being described to me. "You know she has a red car with a ding in the side and her hair is sort of down to here and you were talking to her outside the library the other day. " Right. I still do not know.
"Oh come on Cat! You must know who I mean! She's got this sort of funny..."
The explanation goes on and eventually we sort it out. I do not know the name of the person being described and I certainly would not have called her a friend. She is a perfectly pleasant person but she is a casual acquaintance - if that. As we both use the same library and the same supermarket we recognise one another's faces. We can chat about the weather, something we have read or what her son did at school or her daughter did at pre-school. I have no idea where she lives. I know she buys the expensive 'organic' milk and likes to use the Farmers' Market at the Showgrounds on Sundays. I do not know much else about her.
But, according to the other acquaintance, she is a 'friend". I have been puzzling over this ever since. What makes someone a friend rather than an acquaintance or someone you are friendly with?
I count as friends people I have never physically met. We usually have a lot of things in common. We might have shared experiences. We may have similar interests. I suspect that, at very least, we could enjoy a cup of tea or coffee together. I wonder if this might be the dividing line for friendship v acquaintanceship? I do not know. I doubt it but it may be a marker.