What is the crop factor? What is lens equivalency? If you had a full-frame sensor and your cousin had an APS-C-sized sensor, how would you explain the differences between the cameras and lenses to your cousin?
Crop factor is that a smaller lens will create a more zoomed in image when the focal length is the same. Lens equivalency is the crop factor times the lens focal length. The differences between the cameras and the lenses are that when they are at the same focal length, the field of view is different. In order to create the same field of view, you would have to either use a longer lens on the full frame or a shorter lens on the APS-C-sized sensor. But, focal length will deepen when using a longer lens so you will have to increase the f-stop and then the gain to make up for the change in exposure, both based on the crop factor.