Web typography refers to the use of fonts on the Web. When HTML was first created, font faces and styles were controlled exclusively by the settings of each web browser. There was no mechanism for individual web pages to control font display until Netscape introduced the tag in 1995, which was then standardized in the HTML 2 specification. However, the font specified by the tag had to be installed on the user's computer or a fallback font, such as a browser's default sans-serif or monospace font, would be used. The first Cascading Style Sheets specification was published in 1996 and provided the same capabilities.
The CSS2 specification was released in 1998 and attempted to improve the font selection process by adding font matching, synthesis, and download. These techniques did not gain much usage, and were removed in the CSS2.1 specification. However, Internet Explorer added support for the font downloading feature in version 4.0. Font downloading was later included in the CSS3 fonts module, and has since been implemented in Safari 3.1, Opera 10 and Mozilla Firefox 3.5. This has subsequently increased interest in web typography, as well as the usage of font downloading.
In CSS, authors specify font characteristics via a series of font properties like 'font-family', 'font-style', and 'font-weight'. In CSS1, all fonts were assumed to be present on the client system and were identified solely by name. Beyond the properties mentioned above, designers had no way to propose other fonts to the user other than generic default fonts.
Web-safe fonts are those fonts likely to be present on a wide range of computer systems, and are used by web content authors to increase the likelihood that content will be displayed in their chosen font. If a visitor to a website does not have the specified font, their browser will select an alternative — in the case of dingbat typefaces the alternative is unlikely to have similar characters at the same code points.
In order to give web designers some control over the appearance of fonts on their web pages even when the specified fonts are not available, the CSS specification also allows the use of several generic font families. These generic families are designed to split fonts into several categories based on their general appearance. These generic families are commonly specified as a last resort if none of the fonts specified by the author are available.
Original Text from http://en.wikipedia.org/wiki/Web_typography