Chapter 01: Camera and sensor types
All lenses have a given focal length, for example 100mm. This equals 100mm on a full format image sensor. Crop factor describes how much smaller a camera’s sensor is compared to a 35 mm full‑frame sensor, and therefore how much narrower the field of view becomes with the same lens.
Crop factor is the ratio between the diagonal of a 35 mm full‑frame sensor and the diagonal of the sensor you are using. Example: Most APS‑C has a diagonal around 28.2 mm, while a full format is 43,3mm. To do the math of the crop factor: 43.3 / 28.2 ≈ 1.5×. This ratio tells you how much of the lens’s projected image the sensor “crops away”. A lens stated at 100mm is 100mm on a full frame camera body but equals 150mm on an APS-C camera body and 200mm on a m4/3 camera body.
If you want to see common crop factors I refer to this article at Wikipedia.