Well, i'm thinking there is many OS Server out there like Win**** Server, Ubuntu Server, Red Hat, Centos, etc. What i want to ask is the difference between all of OS Server that would make you want to use it, example for me Free and Easy to use is very important thing. But i wish someone in here could answer in more technical way.

Win* Server - you pay MS taxes forever.
Linux Server (Ubuntu, CentOS, et al) - you own it. RHEL charges for support (probably cheaper than MS), but CentOS and Scientific Linux are free clones of same. For production servers, CentOS, Ubuntu Server, or Scientific Linux are great choices.