Sometimes you will be given a dedicated server or a VPS and you won't know exactly which flavor or distribution of Linux it is running.

Here is how you can find out when logged in through SSH:

# uname -a

This generally won't tell you as much as you'd like.

# cat /proc/version

This will generally tell you more than you'd like.

Once you know that you are running Red Hat for example, you can get to the point with:

# cat /etc/redhat-release

Or on Debian:

# cat /etc/debian_version