Basically a tougher version of a racer, that allows for wider off road tyres. These bikes are popular with those who want a 'do everything' bike that is still performance oriented (unlike most hybrids). If you want light weight, however, it is unlikely in this category; compared to a racer anyway. Cyclocross bikes come in a couple varieties: some have commuting and a 'do everything' construction, others have a cyclocross race course as their purview.
Our favourites are below but we have a lot more options available to us. Come see us in store to discuss what you want.