As I understand it, steampunk is sort of like Victorian Science Fiction. You take the 19th century (a time when Steam engines were still widely used) and combine it with some modern or even futuristic elements.
A lot of people who are into this are REALLY into it, so they wind up using it as a wedding theme. To see how it might translate to wedding decor, check out this amazing wedding cake from Venus Cakes in the UK.