I found this out really recently, or maybe, I finally paid attention to the fact, for as the reality set in, it really, really hurt.
So, I wanted to try and establish, at least for me, the difference between a friend and an acquaintance.
For two reasons: so I don't have to feel so bad and so deeply betrayed ever again, and, probably even more importantly, I don't place too much responsibility on someone who never wanted it in the first place.
Friend versus acquaintance can be easy to confuse as your relationships develop, but I think a lot of it depends on the environment in which you know them. Say, for example, you are involved in a sport and you know them through that area of your life.
For me, friends cross over in to all areas of your life. Not always physically, but emotionally and spiritually. Acquaintances are corralled in one area of your life, with an occasional straying to a restaurant or meeting outside, but nothing consistent and those occasions normally happen on the heels of some kind of event which is somehow connected to the original environment you met them in.
Acquaintances don't really know you, they know you as something they identify with, which is determined again by the environment that you met in, but they don't know you, and they don't care to. An acquaintance will resist getting too many details about other areas of your life. They will absorb enough to be polite and to seem "friendly" but not enough to be tied in too much outside of their already established comfort...