Although there are, a number of ways to label what is effectively the same font, CSS makes a valiant attempt to help user agents sort out the mess. After all, what we think of as a “font” may be composed of many variations to describe boldfacing, italic text, and so forth. For example, you’re probably familiar with the font Times. However, Times is actually a combination of many variants, including TimesRegular, TimesBold, TimesItalic, TimesOblique, TimesBoldItalic, TimesBoldOblique, and so on. Each of these variants of Times is an actual font face, but Times, as we usually think of it, is a combination of all these variant faces. In other words, Times is actually a font family, not just a single font, even though most of us think about fonts as being single entities.
In addition to each specific font family such as Times, Verdana, Helvetica, or Arial, CSS defines five generic font families:
These fonts are proportional and have serifs. A font is proportional if all characters in the font have different widths due to their various sizes. For example, a lowercase i and a lowercase m are different widths. (This book’s paragraph font is proportional, for example.) Serifs are the decorations on the ends of strokes within each character, such as little lines at the top and bottom of a lowercase l, or at the bottom of each leg of an uppercase A. Examples of serif fonts are Times, Georgia, and New Century Schoolbook.
These fonts are proportional and do not have serifs. Examples of sans-serif fonts are Helvetica, Geneva, Verdana, Arial, and Univers.
Monospace fonts are not proportional. These generally are used to emulate typewritten text, the output from an old dot-matrix printer, or an even older video-display terminal. In these fonts, each character is exactly the same width as all the others, so a lowercase i is the same width as a lowercase m. These fonts may or may not have serifs. If a font has uniform character widths, it is classified as monospace, regardless of the presence of serifs. Examples of monospace fonts are Courier, Courier New, and Andale Mono.
These fonts attempt to emulate human handwriting. Usually, they are composed largely of curves and have stroke decorations that exceed those found in serif fonts. For example, an uppercase A might have a small curl at the bottom of its left leg or be composed entirely of swashes and curls. Examples of cursive fonts are Zapf Chancery, Author, and Comic Sans.
Such fonts are not really defined by any single characteristic other than our inability to easily classify them in one of the other families. A few such fonts are Western, Woodblock, and Klingon.
In theory, every font family a user could install will fall into one of these generic families. In practice, this may not be the case, but the exceptions (if any) are likely to be few and far between.
src: CSS: The Definitive Guide, 3rd Edition