Basically everything I would say about "what is it ?", I've already said to someone on page three of the Steampunk Thread
It's basically "What if the future happened in the past ?". You can read about it in detail on Wikipedia : http://en.wikipedia.org/wiki/Steampunk
Why cosplay it ? Well... why cosplay anything ? Because people want to, I guess.
I'm not really "into it" - but I created a character to go along with friends'. It's fun (even if I'm not done and probably won't be forever). It also lets people play with historical themes without the pressure of having to "do it right" - I'm not suggesting people shouldn't research what they do and incorporate historical techniques and design (in my opinion, steampunk with research put into it looks sooo much better to me than pure fantasy steampunk) - but it lets you play around and have more freedom in design and material and adornment.