There are around 200 species. Probably the most commonly grown are probably H. rosa-sinensis (tropical hibiscus). H. syriacus (a hardy woody shrub, sometimes called rose of Sharon--that common name is also applied to other plants), H. mutabilis (common in the South of US, common name - Confederate rose, can be shrubby, behaves more like an herbaceous perennial farther north) and H. moscheutos (a hardy perennial that dies back to the ground in winter--huge showy flowers.
There is a list of species on Wikipedia and links to describe some of them. http://en.wikipedia.org/wiki/Hibiscus
