Mac rookie asking for a little help

Hello everyone I'm new to Mac and new in here so any help is greatly appreciated. I ordered my MBP last summer and I had been a heavy pc user my entire life until then, thus I need some help. I noticed that my hard drive already has about 100 gigs worth of data on it and I don't know what more than half of it is... I have put a maximum of 45 gigs worth of music, movies, pictures, applications, ect. onto it and, correct me if I'm wrong but, the computer didn't come out of the box with more than 50 gigs worth of junk on it. I have noticed that there is 3 different "Library" files; one in the system folder (small) one in the users folder (35 gigs) and another on the HD folder (35 gigs). My question is how is it possible for this much space to be used up already? More specifically, why is there an additional 35 gigs worth of random data in the latter of the three? More over, if the Applications Folder contains the applications, the Users Folder contains all of my personal data/ files, and the System File contains the system files, what is the point of having a 35 gigabyte collection who-knows-what sitting there wasting space and slowing my computer down?

well i have library folders that are pretty huge too...looking forward to an answer myself