Linux is free, and released under the GNU General Public Licence. The C source code is also available freely for you to view or even modify if you're feeling brave. However there is no need for you to do this unless you want to.
Most people obtain Linux by downloading a ``Linux distribution''. These are pre-packaged Linux installations which include everything you need to run Linux, and a selection of programs from the thousands available. The differences between the distributions are often small, and you should not normally need to worry about what distribution a machine is using. The most common distributions in the University of York are RedHat and Suse.
Modern Apple Macintoshes run MacOS X, which is based on a UNIX distribution called FreeBSD. The University also has some Sun machines running Solaris, Sun's UNIX operating system. Much of this talk also applies to these machines.