If you want kernel version information, use uname(1). For example:
$ uname -a Linux localhost 3.11.0-3-generic #8-Ubuntu SMP Fri Aug 23 16:49:15 UTC 2013 x86_64 x86_64 x86_64 GNU/Linux
Distribution Information
If you want distribution information, it will vary depending on your distribution and whether your system supports the Linux Standard Base. Some ways to check, and some example output, are immediately below.
$ lsb_release -a No LSB modules are available. Distributor ID: Ubuntu Description: Ubuntu Saucy Salamander (development branch) Release: 13.10 Codename: saucy $ cat /etc/lsb-release DISTRIB_ID=Ubuntu DISTRIB_RELEASE=13.10 DISTRIB_CODENAME=saucy DISTRIB_DESCRIPTION="Ubuntu Saucy Salamander (development branch)" $ cat /etc/issue.net Ubuntu Saucy Salamander (development branch) $ cat /etc/debian_version wheezy/sid