The Difference Between Full Frame vs. APS-C
Full-frame and APS-C formats indicate the sensor's physical dimensions, entirely different from pixel count. A full-frame sensor has 36mm by 24mm in size based on the traditional 35mm film format. An APS-C sensor is 1.5 times smaller, 25.1mm by 16.7mm, and named after the Advanced Photo System type-C film format, hence its abbreviation.
35mm film has historically been the more popular format due to its near-perfect size for capturing almost anything under the radar. In the analog world, it's much easier to carry a 35mm camera than a medium or large format camera, no? While more compact, they're seemingly large enough to produce high-quality photos making them a highly desirable piece for professional and amateur photographers alike.
This term – full-frame – was defined in contrast to more minor, or APS-C, camera sensors. A full-frame lens is roughly equivalent to a 35mm film frame, while an APS-C sensor is slightly smaller. When you mount a full-frame lens on a camera with an APS-C sensor, you will get a crop factor; your camera's APS-C-size sensor magnifies the scene to produce an image that will match the lens's full-frame image circle.
The effect is that a 50mm full-frame lens mounted on an APS-C body with a 1.5x crop factor will capture a field-of-view that is the same as a 75mm on a full-frame body.