I see friendship as a variable thing. I think there are various levels to it even if we were to drop off the "buddy" thing or the "acquaintance" and not consider them part of the tree.
For me I'll assign a random term to the highest level as "true" friend". Only a label for explanatory purposes.
I think most of us have FEW "true friends" in our lifetime. Some may never have any unfortunately. Because it seems the building blocks of this require a level of experience (which Caveman mentioned), openness and vulnerability, raw honesty and trust that we do not extend to many people.
The better you are at extending yourself in these ways the better your chances of developing that close bond. And whatever risks there are with that.
But if you get there, it seems there is a sometimes unspoken understanding between you, that you will always be there for each other when needed. Stand together through both the celebrations and the crisis. And a level of trust that you will always be totally honest with each other - to the point of brutality. The one person you can always depend on to tell you when you are full of sh%$ and headed down a dark road. A hand extended - but with deep enough respect to honor your choice without nullifying that bond that exists. Willing to pull you from that dark chasm - but not jump over with you.
That level of bonding is special and rare. And maybe it's the highest desirable level of a "relationship" while all the time being that much more difficult to achieve because of the artificial framework so many people place around the term (relationship).
Which is why I frequently advise people to embrace the "friendship" part and not to let that slip away as you try to travel together in "relationship".