What is an M or em space?

Em space is a relative unit of measurement used in typography. Once the size of an em is established, the size every other character and other typographic units of measurement can be determined as a fraction of em. The reason why em is used is because the letter "m" is the only letter that has relatively the same width in its capital and lowercase form. There is also something called en space. En is usually about half the width of an em so if the em space is 12 points, the en space is usually about 6 points.