Advanced Photo System type-C (APS-C) is an image sensor format approximately equivalent in size to the Advanced Photo System "classic" size negatives. These negatives were 25.1 × 16.7 mm and had an aspect ratio 3:2.
Sensors meeting these approximate dimensions are used in many digital single-lens reflex cameras, in addition to a few large-sensor live-preview digital cameras (such as the Sony DSC-R1, Sigma DP1 and Leica X1) and a few digital rangefinders (e.g., the Epson R-D1). 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. 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.
Read more about APS-C: Advantages and Disadvantages, Brand Designations, Multiplier Factors, APS-C Lens Formats