Cyclocross is a form of bicycle racing consisting of short laps over many differing terrains, typically including grass, sand, mud, pavement, rocks and anything the course designer finds at the race site. Each lap also usually includes steep run-ups, barrier obstacles forcing the rider to get off the bike.
Bicycle companies are now producing cyclocross specific bikes, but beginners can use a mountain bike at some races. A road bicycle can be converted into a cyclocross bike. New cyclocross bikes are light, as you will be lifting and carrying your bike often, they also have lots of clearance around the brakes, to facilitate wider tires with traction plus allowing caked on mud to not hinder forward progress.
The duration of the race will depend on your age and classification, beginners typically race around 30 - 45 minutes, while top level pros go around an hour.