Originally Posted by Arttemis
So you have to pay a monthly fee to play with your friends in Minecraft? And is that on top of XBL Gold?
Not to play with them - all you need for that is the fee for general multiplayer on your system of choice, if any. This monthly fee is to have your world hosted on external servers and available at all times.
It works this way: if you and I like to play together and we are working on a world together, without Realms, one of us has to be the host who has the save file for that world on our console. The host goes online and the guest joins my session. But as soon as the host quits the guest is kicked out. You would have no way to continue building our castle or whatever in your own time if I am not online and loaded into that world and letting you come in to my session.
With Realms the world is just there on the server, independent of specific consoles, save files and single sessions, and anyone with access can jump into it when they want. It's much more flexible that way for having a world you and multiple friends can play any time. We could have group play sessions to build together and separately jump into "work" on stuff on our own or in smaller subgroups.
Of course if your playstyle is more towards logging on at the same time anyway and you wouldn't want to go into that world without the other person, you can just do normal multiplayer.