You can jazz up your documents — or make them a little more serious — with different fonts. To a computer user, font means typeface — what the text characters look like. Although professional typographers will scream at my generalization, I’ll go with that definition for now.
Tens of thousands of different fonts are available for the Macintosh. You don’t want to use the same font for both a garage-sale flyer and a résumé, right? Luckily for you, OS X comes with hundreds of fonts. Some are pretty predictable, such as Times New Roman (the font of this paragraph), but OS X gives you some artsy ones, too, such as Brush Script. If you really get into fonts, you can buy single fonts and font collections anywhere you can buy software. Plenty of shareware and public-domain fonts are also available from online services and user groups. Some people have thousands of fonts. (Maybe they need to get out more.)
The preinstalled fonts live in two different folders, both called Fonts. One is in the Library folder at root level on your hard drive; the other is in the Library subfolder within the System folder.