How to Check Linux Version and Distro

Who run more than 10 Linux servers with different version of Linux Distro? We all know there are many type of Linux distro such as Ubuntu, Centos, Gentoo, Fedora, debian, OpenSUSE and etc. Every single distro runs on different version and etc. Sometime I am having problem to remember what Linux distro is running on my DNS server or Mail Server, and what Linux version is it on now. There are two methods to find linux version on your server.

To check Linux distro on your server

To check Linux Version and distro on your server