Web Design & Development Guide
Web-safe fonts
Web safe fonts are the fonts guaranteed to be present on all or most computer systems, and hence can be relied on to display consistently on webpages. Typically, not having a particular font installed will cause reversion to some system default font, or, in the case of alternate character encodings, "garbage" or meaningless characters to be displayed.
CSS Generic Font Families
The most safe option is to use the generic font families as defined by the CSS. These are: sans-serif, serif, monospace, cursive and fantasy. One of them should always be specified as a last resort for every font-family CSS property.
Microsoft's Core Fonts for the Web
Microsoft has started a Core fonts for the Web initiative. However, despite the name, only some of the fonts defined there are web-safe indeed. These are: Arial, Courier New, Times New Roman, Verdana, Trebuchet MS (more recently) and Lucida Sans (to some extent).
Web-Safe Unicode Fonts
Only two fonts available by default on the Windows platform, Arial and Lucida Sans, provide a wide Unicode character repertoire. A bug in Verdana (and the different handling of it by various user agents) hinders its usability where combining characters are desired.
Dingbats
No dingbats font, including Webdings, can be considered web-safe.