Cracked minecraft servers also known as offline servers are servers that sometimes allow players to play without having an official minecraft account. There are also some servers that combine the offline and premium mode, allowing you to play anywhere and anytime the way it suits you.
If you want to play on an offline server you're going to want to find a cracked launcher and there are plenty floating around the internet. I recommend using this minecraft launcher as it comes with a preloaded list of servers that are pretty fun way to start playing. All that's left is to choose a name that's not yet used, you might have to try a few as there are quite a bit of players already, but when you find one you like you can just start playing without worrying about anything else. If however you want to find a good server for yourself i'd advise you to head on over to this list of minecraft servers, where you can find just about any type of minecraft private server you want.
See you in game.