Advanced Photo System type-C (APS-C) is an image sensor format approximately equivalent in size to the Advanced Photo System “classic” negatives of 25.1 × 16.7 mm, an aspect ratio of 3:2.
Such sensors exist in many different variants depending on the manufacturer and camera model. All APS-C variants are considerably smaller than 35 mm standard film which measures 36×24 mm. Because of this devices with APS-C sensors are known as “cropped frame”. Sensor sizes range from 20.7×13.8 mm to 28.7×19.1 mm. Each variant results in a slightly different angle of view from lenses at the same focal length and overall a much narrower angle of view compared to 35 mm film. This is why each manufacturer offers a range of lenses designed for its format.
Using the same resolution, quality of image from APS-C sensor is higher(Especially in low-light scenarios) when compared to smaller sensor, due to its pixels having a larger size which allows it to collect more light. APS-C sensor can also have higher resolution without any significant decrease in image quality.
As APS-C sensor is smaller than Full-Frame sensor, they increase the apparent focal length of the lens meant for Full-Frame sensor, which results in images appearing to be more “zoomed in”. If the lens is not perfect, geometric distortions (near the picture edges in particular) may decrease with a smaller sensor.
And of course, camera size, weight and price decrease as sensor size decreases.