Cyclocross was originally developed by/for european road riders who needed a way of staying reasonably fit over the cold/wet winter months. Sanctioned events are typically one hour long and held over a 1-3 mile course, mostly over dirt and grass. There are typically areas were riders need to "bunny-hop" over logs or planks, ride through water/mud, and even dismount and run with their bikes. The routes are typically not as technical as what you might find in mountain bike races/trails. Most cross bikes have a stout frame with 700cx30-35mm knobby tires and cantilever or linear-pull brakes. The bikes are geared for mud/dirt/uphill riding and triple chainrings up front are not uncommon, giving you 27spds to choose from. The sidepull brakes are typically not used because they tend to get clogged with mud much easier than the canti's/linear pulls. Also, a typical road frame is not able to accomodate the wider tires generally used for cyclocross riding. You can use your frame to get started and see how you like riding on dirt, but if you get interested in it you'll need some more specific equipment. Often a touring bike frame can be used for 'cross riding (if suitably equipped), and a cyclocross-specific frame makes a very good touring/commuting/all-around bike as well. Cross-specific frames may be difficult to find at your local bike shop, as they are a specialized item, but often they can order them for you. Be aware that 'cross frames may look similar to typical road racing frames, but the sizing is different due to changes in geometry - see the threads below about 'cross frame sizing!