Kernel Version
To find out the version info for the running kernel:

$ uname -a

This will give you something like:

Linux t.example.com 2.6.18-128.1.14.el5 #1 SMP Wed Jun 17 06:40:54 EDT 2009 i686 athlon i386 GNU/Linux

Ubuntu
On the ubuntu terminal, type:

$ cat /etc/lsb-release

Which will give you something like this

DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=9.10
DISTRIB_CODENAME=karmic
DISTRIB_DESCRIPTION="Ubuntu 9.10"

CentOS and Red Hat Linux
On the terminal, type:

$ cat /etc/redhat-release

This will give you something like

CentOS release 5.6 (Final)