... wat is this?

You were in the trading card beta, duh :P

It's an emoticon, you can insert it into steam chat by typing :tradingcard: or selecting it from the emoticon menu. It's like an animated smiley or symbol, Steam decided to make those collectible too.

'tis a collection of pixels

that is a screenshot of your steam inventory

I can confirm it is not a hippo.

you can type :tradingcard: in steam chat with friends to show it off.

these are trading cards.
its still in Beta but you can already use it.

its like the achievements in games, you can now also get trading cards for playing, some cards are worth a lot and if you trade them you can recieve some other good items or cards for it.

An emoticon.

It's a reward for participating in the Steam Trading Card Beta.

It's a tradable, marketable, common emoticon.

People cannot read.

why don't you literally just hover over it and read >.>

Another steam gimmick / whatever the hell is going on in their heads.

I read it and wondered what exactly the hell it was, because it looked like it was broken.
I think I had another item like it before which appeared the same, but I can't remember what it was.