When designing websites, professionals will use a standard set of fonts common to most computers. These are commonly referred to as "browser safe fonts".

You might be thinking: "Why just use these, I have a lot more fonts on my computer?"

Well, not everybody will have all those fonts installed. Windows, Macs, Smart Phones, Unix machines and other device and platforms use variations of those fonts. We need to use a font that is common to all. If we do not, in some cases vistors to your website may not be able to read your website clearly.

You can put your text into raster graphics - and thereby use any font you like. However this is not recommended for SEO website.

New web standards will eventually enable rendering of other more exotic fonts. However, until these standards are widely adopted by all visitors, designers will still need to keep in mind these browser safe fonts.

Serif Fonts

Serif fonts have small lines tailing from the edges of letters and symbols...

Sans-Serif Fonts

San-Serif fonts do not have small lines tailing from the edges of letters and symbols. "Sans" is french fro "without"...

Monospace Fonts

These fonts have the same character spacing. They are useful for basic text tables, or for text only formatting.

References:

http://www.w3schools.com/cssref/css_websafe_fonts.asp