good question, and maybe not so small! I think that it has to do with several different things that overlap, and that's why it's harder to define rather than experience. For example
1) What it is. Streetwear is mostly jeans, tshirts, hoodies, and sneakers. Of course, there is an obvious overlap, that is a fairly recent phenomenon. I think it results from men now willing to pay a premium in order to look casual, hence a rush of designers to make the aforementioned items. Still, you won't find tailored stuff and boots/shoes in streetwear (with a few exceptions, I guess). One of the things I love about designer clothes is that you can so easily dress it up and down, often by simply changing a single item, and sometimes even changing one's perception.
2) Where it comes from. Some streetwear has its roots in cultures of skateboarding, surfing, rap, whatever (sorry to sound like a clueless academic here, lol). Usually designers have a more personal point of view rather than a broad cultural one - it is their own interpretation of culture, as opposed to a mere reflection.
3) What it's made of. This is self-evident, although again there is an overlap. That's where Number (N)ine comes in, I guess - their stuff is well made, usually from luxurious materials (good wool, cashmere, cotton/cashmere, etc).
4) Where it's sold. This is pretty obvious, and I am putting it in simply to draw a clearer picture.
I am sure others will chime in with a different perspective.