From the perspective of an outsider who knows very little about 'Steampunk culture', Steampunk is all about machines and technology that can accomplish sci-fi-esque futuristic results using technology that was available in the 17th-19th century century, namely things with big, showy copper pipes used to conduct steam.
There's a indie video game that I got as part of the Humble Indie Bundle back around Christmas that I have yet to put much time into called Jamestown. The idea behind it is that the British colonized Mars during the 17th century. It is a top-down shooter similar to 19XX (a 90s arcade cabinet that I have fond memories of). There isn't anything specifically Steampunk-ish about it, but I use it as an example because it has a similar theme: What if folks way back when had access to technology that allowed them to do all sorts of cool stuff like fly around and go to other planets? That seems to me to be the core of Steampunk.
Now, Steampunk is a lot of other things that I am sure I am missing. It is an entire subculture with a million different facets that only people thoroughly ingrained in it really know, but to me, as an outsider, it comes down to one thing: possibilities.