Brief crash course if the Library inheritance system:
There are three levels of Library folders:
The first is what Apple gives you. The contents here apply to all users. Users should *never* touch what is inside this folder. It should always remain "clean" and safe. Contrary to what ast3r3x said, you should never enable and use the root account to modify the contents here. Only experts
with the system should even consider enabling the root user. Using the root user opens up vulnerabilities and may cause odd Finder behaviors for other users among other things.
The second should only be writable by administrative users. Like System, its contents apply to everyone, but if there are ever problems and the system has to be booted into safe mode for repairs, this Library will be ignored and only the known safe items in the System folder will be loaded.
The third is the preferred folder for installing things on a per-user basis. Contents of this folder only apply to the user it belongs to. It too would be ignored in safe mode.
Does that help?