Image sensors are the device in digital cameras that convert the optical image into digital information. The two main specifications of image sensors are resolution (The vertical and horizontal pixel count, most often cited in megapixels) and crop factor.
Crop factor is the size of the sensor relative to the size of the 35mm reference frame. This measurement is used to determine (roughly) the performance of the sensor, as well as the amount of image that is cropped when using SLR lenses on film cameras. On most DSLR cameras, this factor is usually between 1.3x and 2x. The amount refers to the magnification of the image compared to a 35mm-sized sensor, where 1x would be equal to 35mm, and 2x would be half the size.
In general, sensors that are larger (crop factors closer to 1 or lower) perform better, as each pixel is accordingly larger, and therefore better capable of capturing light.
Most digital cameras have sensors of uniform sizes, known as formats. The most common format is APS-C (Advanced Photo System type-C), which is a specification for sensors that have a dimension of approximately 24 x 16 mm, and a crop factor of 1.5.
Canon has a unique size of APS-C with a crop factor of 1.6x, as well as a larger APS-H sensor with a crop factor of 1.3x.
Some high end DSLR cameras have “full frame” sensors, which bear the same dimensions as a frame of 35mm film (36mm x 24mm).