The crop factor is often the ratio of the dimensions of a camera's imaging area compared to a reference format. Generally speaking Crop Factor is applied to digital cameras, relative to the old 35 mm film format as a reference point.

In simple terms some sensors give you more (or less) zoom all else equal.

In other words with the same lens and body you may get a 1.5x zoom if you use an APSC sensor vs on 35mm.

Here is a video on this topic:

video by Tony Northrup

Here is a spreadsheet with some common crop factors.

See Also:
Aspect Ratio
Photography Glossary