[jameshart]

Right, classes are a way to explain to the computer 'there's gonna be a bunch of objects that are all "chairs". You'll be able to treat all of them in a similar kind of way.' But you should consider that starting with classes might not be the only way to do that.

Inform7, in its beautifully literal (not to say literate) way, captures this very simply with the way it defines classes (what it calls 'kinds'):

     A chair is a kind of thing. A chair can be comfy or hard.
     Your favorite armchair is a comfy chair in the living room.

But to be fair, Inform7 is object-oriented because it's a language for describing worlds (interactive fictional worlds, specifically) that are made of objects, and the utility of the metaphor for building, say, a website, may not be as strong.

Other languages might use other techniques to capture 'chairness' without having to start off by describing the class of 'chair'. Maybe you define the protocols chairs support (sittability?), or you just rely on dynamic typing and write code that just assumes whatever is passed to it is a chair and can be sat upon, without the code ever needing to be able to divine an object's chairness. That's useful! you can sit on things that aren't chairs, after all. If you can only build objects from classes, though, you can get yourself into the situation where you find yourself having to find a way to compose a 'chair' instance into your 'bed' class so you can allow someone to sit on the bed without having to copy paste code.