Example: font-family: "Arial", verdana, arial, helvetica, sans-serif;
When you specify a font family use the preferred option at the beginning of the list, followed by a second option (then third and fourth options if wanted, and ending the list with a font category such as sans serif.
When a browser displays text it is limited to using the fonts installed on the computer, so by using a font family you are supplying alternatives to your preferred option. The computer searches for the first option, and if it's found, that font is used. If the first option isn't found, the computer searches for the second option and uses that instead, and so on through the list of options. If none of the specified options are found the computer uses the first available font belonging to the font category.