On my powerbook whem I double click on the Mac HD on the desktop their is a library folder that takes up 10 gb of space . The problem is I already have that folders in my home folder but one of them hase more items than the other one, my question is can I delete one of those folders or do I need them because it seems as if I already have them in my Home folder and I really need that extra 10gb. Please help.
The first Library you mention is the System one, do not try to delete anything in there without knowing what it is for or you will end up will Apps that no longer work, the second one is your personal Library.
The difference between them is that when you install large Apps like Final Cut Express it places the media you need (Filters, Transitions, Loops etc) into the System Library so that every user can access them whereas when you are running Apps the user Library is used for things that are speciific to your user (like particular filters you may have included in your own projects).
Other Apps do the same so System Library contains plugins for browsers, bits of Open Office or MS Office, Photoshop bits and pieces, GarageBand loops etc etc.
If you have no more use for some of these files, like for example you have deleted MS Office, then you can clear out the files but be very careful. Plugins and helpers in particular are sometimes used my many Apps.
Apple have made an effort to make the file usage pretty obvious, i.e the names of the folder are normally a dead giveaway as to what they are for and which App put them there but some third partys are not so user friendly.
btw, if something says it is a "Framework" or is in a folder named "Frameworks" leave it well alone. These are libraries of code needed by applications, unlike media files if these are not present you could end up with all sorts of strange behaviour from the system.
btw2, if you install something and it askes for the admin password that is a dead giveaway that it is installing things in the System Library.