A full-frame camera has a sensor that's roughly the same size as a 35mm film frame, whereas a crop sensor is smaller. The difference impacts the 'crop factor' which affects the effective focal length of lenses. For instance, a 50mm lens on a crop sensor camera with a crop factor of 1.5x will have an effective focal length of 75mm. When choosing a lens for a crop sensor camera, you need to account for the crop factor. The sensor size can also impact the depth of field and low light performance, with full-frame cameras generally performing better in these aspects.