We generally provide two main type of Linux distros

Redhat based (.RPM) : Fedora, Centos
Debian based (.DEB) : Debian, Ubuntu

If you are unsure what distribution you are running you can find out using 2 methods

1 -  Using the Plesk control panel :-
log-in to Plesk (as admin)
click on the server link (in the left hand menu)
click statistics

In the 'version' line you will see something similar to:-

psa v8.2.0_build82070807.17 os_Debian 4.0

The distribution can be identified after the OS.

The main ones are

Debian based :-
os_Debian 3.1 – Debian Sarge
os_Debian 4.0 – Debian Etch
os_Ubuntu 6.06 – Ubuntu Dapper

Redhat based :-
os_Fedora Core 4/ 5 /6
os_Centos 4 / 5


2. The second way of identifying what OS you are running is via the command line
(this provides more detailed information)

log-in as root to SSH
Type the following command

lsb_release -a
i.e on a Debian etch (4) system the output will look like this:-

Distributor ID: Debian
Description:    Debian GNU/Linux 4.0r1 (etch)
Release:        4.0r1