A fat quarter of fabric is a piece cut from 1/4 of a yard. A fat quarter of fabric is a rectangle that most commonly measures 18 inches by 22 inches. It differs from a regular quarter in that the full yard is cut in half both lengthwise and widthwise.
A regular quarter is cut in quarters lengthwise only and maintains the width of the full yard.
This cut is ideal for quilting, according to About.com. The fat quarter the most common speciality cut, as it gives the user the ability to cut larger pieces for quilting that are impossible with a regular quarter cut.
A fat quarter is also the easiest way to get a fat eight, which typically measures 9 inches by 22 inches.
Many fabric stores offer pre-cut fat quarters for ease of use.