There are several places OS X looks for fonts. They are (I don't remember the order it looks in):
System > Library > Fonts
Library > Fonts
Users > *Your user name* > Library > Fonts
System Folder > Fonts
Network > ? > Fonts
The fonts in the System and Library folders are available to all users on your computer using OS X. Those in System Folder (if you have OS 9, you will have this) are available to everybody in both OS X and Classic (boot or environment). The fonts in individual user accounts are only available to those users, and the fonts on the network are, naturally, available to anyone on the network. So place them according to who you want to be able to access them. (All paths except for Network start at the root of your hard drive.)