When supporting systems you have inherited or in environments that have many different OS versions and distributions of Linux. There are times when you simply don't know off hand what OS version or distribution the server you are logged into is.
Luckily there is a simple way to figure that out.
Ubuntu/Debian
$ cat /etc/lsb-release
DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=13.04
DISTRIB_CODENAME=raring
DISTRIB_DESCRIPTION="Ubuntu 13.04"
RedHat/CentOS/Oracle Linux
# cat /etc/redhat-release
Red Hat Enterprise Linux Server release 5 (Tikanga)
Catchall
If you are looking for a quick way and don't care what the output looks like, you can simply do this as well.
$ cat /etc/*-release
DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=13.04
DISTRIB_CODENAME=raring
DISTRIB_DESCRIPTION="Ubuntu 13.04"
NAME="Ubuntu"
VERSION="13.04, Raring Ringtail"
ID=ubuntu
ID_LIKE=debian
PRETTY_NAME="Ubuntu 13.04"
VERSION_ID="13.04"
HOME_URL="http://www.ubuntu.com/"
SUPPORT_URL="http://help.ubuntu.com/"
BUG_REPORT_URL="http://bugs.launchpad.net/ubuntu/"