As I mentioned before, Steampunk is in fact a subgenre of speculative fiction, however that doesn't say much its characteristics and what makes it what it is.
One thing I've noticed when talking with people about Steampunk is that it represents different things for different people. However, for most folks, some things are quite common, so let's see what else can we attribute to Steampunk:
First of all, it is in fact speculative fiction, which means that one might ask what would happen if a particular story or even played out in a different way? Imagine your watching a movie and it ended. At some point you might start daydreaming about what if the ending were different, what if characters took other actions that led to other developments. So basically the storylink always takes place in an alternative universe, similar to ours, yet not quite the same.
Also steampunk includes anachronistic technology, and by this we mean technology that normally wouldn't exist in that particular era, even if it is common right now. For example computers are very common these days, but they wouldn't be even existing in the 18th Century. Or imagine someone using an iPad in the 19th Century.
The setting is usually the 18th or 19th Century Victorian England. Alternatively you might find steampunk movies or novels set in the American Wild West. However where it differs from Victorian era fiction as such as in the fact that it doesn't really depict that time in its true light with dirty streets, non-hygienic settings, poor people and all sort of incurable diseases which now are cured with simple pills. Steampunk fiction idolizes Victorian era, romanticizes and gives it attributions of the current day and time, which are far from being accurate at all.
Also where it comes extremely close to dystopian fiction, steampunk usually contains an element (or a character) that is out of the norm, that rebels or is seen as very different in the way they act, dress or look like.
Image Source: Creative Commons (depicting G.D. Falken, one of the most popular Steampunk authors of all time)