In HTML and XHTML, a font family is a group of similar fonts.  There are 5 predefined generic families:

Serif:	abcdefghijklmnopqrstuvwxyz
Sans-serif:	abcdefghijklmnopqrstuvwxyz
Cursive:	abcdefghijklmnopqrstuvwxyz
Fantasy:	abcdefghijklmnopqrstuvwxyz
Monospace:	abcdefghijklmnopqrstuvwxyz

(Under Windows API, they are identified as Roman, Swiss, Script, Decorative and Modern, respectively).

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There may be many other font families;  however, you cannot expect them to be universally supported.  Just see what your browser can make out of it:

Times:	abcdefghijklmnopqrstuvwxyz
Roman:	abcdefghijklmnopqrstuvwxyz
Garamond:	abcdefghijklmnopqrstuvwxyz
Palatino:	abcdefghijklmnopqrstuvwxyz
Antiqua:	abcdefghijklmnopqrstuvwxyz
Minion:	abcdefghijklmnopqrstuvwxyz
Helvetica:	abcdefghijklmnopqrstuvwxyz
Swiss:	abcdefghijklmnopqrstuvwxyz
Impact:	abcdefghijklmnopqrstuvwxyz
Script:	abcdefghijklmnopqrstuvwxyz
Decorative:	abcdefghijklmnopqrstuvwxyz
Blackletter:	abcdefghijklmnopqrstuvwxyz
Fraktur:	abcdefghijklmnopqrstuvwxyz
Modern:	abcdefghijklmnopqrstuvwxyz
Courier:	abcdefghijklmnopqrstuvwxyz

The actual looks will depend on you browser and the fonts installed on your system.  Internet Explorer, for instance, always displays Serif and Times as Times New Roman, and Sans-serif and Helvetica as Arial.

Typically font family instructions are placed in Cascading Style Sheets.