I'm confused about this as well, and I consider myself somewhat tech savvy. I've read this tutorial here:
http://wiki.minetest.com/wiki/Setting_up_a_server and I've even run the bash file, which quite honestly the terminal didn't really tell me what it was doing.
it just doesn't answer a lot questions for me? Do I need a domain i.e. linux "LinuxGaming.us?" Is this public? Does it just show up in the minetest list? if so under what name? I don't see a place to put a name, or what setting in minetest determines the name. the world name?
What I want to do is like Sparkly here, I want to set up a private game between my friends, and whoever we invite. Do I just start a multiplayer and give them my IP address? Is that secure? if someone else logs in can I kick them out? I could think of more questions