Basically, OS X replicates the same "Library" structure in many places in the computer:
-in each user folder (~/Library) --> available only to that user, for user-added things
-right at the root of the hard drive (/Library) --> systemwide, for admin user-added things
-in the System folder (/System/Library) --> systemwide, for Apple-provided things only
-in Network (/Network/Library) --> networkwide, for network administrator-provided things
Effectively, for any given user, the contents of all those various Libraries are combined, and the user sees the total resources of all of them.
The whole reason for this is to be able to distribute resources (like fonts, screen saver modules, preference panes, etc) in various scopes -- just one person? everyone on one computer? everyone on the network? -- and to keep the required resources isolated and protected, so they can't be damaged (which is why there is the /System/Library folder, which is only for the basic Apple-provided things).
Does this help?
tooki