I presume you are new to Mac OS X and Macs?
Anyway, the OS is called "Mac OS X 1.05 Leopard", the Mac might be any Mac built since 2000, but I assume you have a PowerMac G4.
To find out, go to

> About This Mac ... >> More Info >>> Hardware Overview >>>> Model Identifier and report back.
To find out how many partitions and HDDs you have inside the PowerMac, if that is what you have, open Disk Utility, located in the Utilities folder, but it can also be found via Spotlight (CMD+SPACE).
Normally you can just drag items off the Finder Sidebar without actually deleting or ****ing something up.
Maybe you enable the Path Bar (View > Show/Hide Path Bar) and click on each item in the Sidebar and see for yourself where they are located.
You can also dupeGuru to look for duplicate files and folder.
But please, please don't delete anything outside the Users folder, except some applications you might not need or take up too much space.
Btw, if you have the Leopard Retail DVD, you could just do an Erase and Install and get rid of all the previous owner's crap.
Also have a look here to learn more about Mac OS X:
Helpful Information for Any Mac User by
GGJstudios