There are three important ones I'll discuss:
- /Library
- /System/Library
- ~/Library
The Library folders are where your applications keep their settings and support files. This is where you'll find preferences, fonts, caches, databases, scripts, and just about any files that
you don't explicitly save yourself. So, why are there three of them? Three words: permissions and inheritance.
/System/Library is not writable by anyone. This contains files that belong to the system and that apply to everything everywhere. This includes the default system frameworks and libraries.
/Library is stored at the top level of your hard drive. It is only writable by admin users. You would store in there files that you want to apply to everything. For example, if you put a font in here, all users will have access to it in their software. If you put a screen saver here, all users will be able to select and use it.
~/Library is located in your home folder (the ~ is unix shorthand for your home, as SonOfSylvanus mentioned). Each user has one and it is only accessible by that specific user. Files you place here will apply
only to you (or a given user). If you put a font here, only you will have access to it.