Photo / Video
APS-C
A camera sensor smaller than full-frame that crops the view about 1.5x for cheaper, lighter extra reach.
A sensor format smaller than full-frame, typically around 23.5×15.6mm (Canon's is slightly smaller). Relative to full-frame, the smaller sensor applies roughly a 1.5× crop factor, so a 100mm lens frames like a ~150mm would — handy extra reach for wildlife and sports. APS-C bodies and lenses are usually lighter and cheaper, which makes them a popular starting point and a capable travel system. The trade-off is that the smaller sensor generally gathers less light, so it tends to have less low-light headroom and a little less background blur than full-frame at the same settings.
What it means when you're buying
APS-C is often the better value for reach, portability and price, while full-frame leads when low-light performance is the priority.