About Linux, the Linux itself is the UNIX-clone free Operating System. The direct competitor to Microsoft's Servers/Workstations. Linux can come in different flavors -- varying on how the directory tree, services and so on was designed.
Redhat is Linux, created by Redhat company to meet customer's specific needs.
There are another companies i.e.:
Slackware
Debian
Suse
etc...
There are SOOOO many different things that can come in Linux's desktop, depending on windowmanager (KDE, gnome, Enlightenment, WindowMaker etc...) and it doesn't matter whether it is Redhat or whatever.