A full-frame camera has a sensor with the same dimensions as standard 35mm film, while a crop sensor is anything less than that and will crop some of your field of view when you shoot.  Full-frame cameras tend to be heavier and more expensive than crop sensors, but usually have a better dynamic range and can produce a shallower depth of field. However, most beginner and intermediate photographers shoot with crop sensor cameras, as they're more affordable and still take impressive photos.