Yes, there are THREE, well more if you have more users....
Mac HD/Library = This is where global additions go. For example if you install screen savers "for all users". Applications can install here to so all users can access the support files, such as Garageband.
Mac HD/System/Library/ = This is, sorta, the "main" library for the system. Apps should not install stuff here. Really this folder does not have much, if anything, that any user interacts with that they can change, like system preferences. Its more of a "services" library that the system owns.
Mac HD/Users/XX/Library = This is items specific to the actual user such as the items in the dock, mail and Safari settings, etc...
When you log-in, all three are sort of merged together to provide one library folder for the system. Or two of them... I forget...
Now, as basic as I have TRIED to make this in leman terms, some one will surly try to say how flawed I am.....

lol