If you're a really curious mind, you won't be satisfied with simply knowing the current release of your RedHat Linux, that's why there's a few more commands you could use to satisfy your interest.
If you simply want to confirm whether you're using a RHEL4, RHEL5 or any of the previous RedHat Linux releases, this is the first place to look:
bash-3.1$ cat /etc/redhat-release Red Hat Enterprise Linux Client release 5 (Tikanga)
RedHat kernel version and type
Next step is to find out the exact Linux kernel version on your system, and also confirm whether it's 64-bit or not:
bash-3.1$ uname -a Linux rhserver123 2.6.18-8.el5 #1 SMP Fri Jan 26 14:15:14 EST 2007 x86_64 x86_64 x86_64 GNU/Linux
RedHat kernel build
For the most curious ones, here's the last command. Use it to confirm who and when compiled the RedHat kernel you're using, and what gcc compiler was used in the build process.
bash-3.1$ cat /proc/version Linux version 2.6.18-8.el5 (email@example.com) (gcc version 4.1.1 20070105 (Red Hat 4.1.1-52)) #1 SMP Fri Jan 26 14:15:14 EST 2007