Okay.. I'm new here. What is this exactly?

I was told this is an private server, My first assumption was that it's for myself and myself alone, but it suggests otherwise.
And, does this actually require me to own the original game? Or is this entirely free? Sorry kek.

Edit: Sorry if this is the wrong place to post. I've got no idea what I'm doing.

Private server means that it's a server not owned by blizzard, in this case "Warmane" owns the servers and they allow up to 12 000 players connect to their "Icecrown" realm / server for example.
The game itself is free to download for anyone just click the "Download" on the homepage.
You then go into your "World of warcraft" folder after the download is finnished - Data Folder - enUS - and inside the "Realmlist" text document you can change the name to whatever Private server you want to connect to, it should be preset to "set realmlist logon.Warmane.com" so it automaticly connects to "Warmanes" server.
Yes, it is that easy.