Which Linux Version, which Kernel version i am running?

To find out what version of Linux (distro) you are running, enter the following command at the shell prompt:
$ cat /etc/*-release
Sample output on RHEL 5:

Red Hat Enterprise Linux Server release 5 (Tikanga)

Sample outputs on Ubuntu v7.10:

DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=7.10
DISTRIB_CODENAME=gutsy
DISTRIB_DESCRIPTION="Ubuntu 7.10"

lsb_release Command

The lsb_release command displays certain LSB (Linux Standard Base) and distribution-specific information. Type the following command:
$ lsb_release -a

Sample outputs:

No LSB modules are available.
Distributor ID: Debian
Description: Debian GNU/Linux 6.0.1 (squeeze)
Release: 6.0.1
Codename: squeeze

How Do I Find Out My Kernel Version?

Type the following command:
$ uname -mrs

Sample outputs:

Linux 2.6.32-5-amd64 x86_64