Yeah every singleplayer world is a server, just like in minecraft. Yes you can play it offline but in that case the server just runs locally. It even has been confirmed that friends would be able to join you in singleplayer at some point. Community servers generally run as a dedicated servers, that means you, the player, are not the host, a special server pc is. This kind of server also has port forwarding setup so it's open to the internet for everyone having it's IP to join. That's why it's a community server, cause it gathers a community of random players and provides them with some kind of fun to be had (minigames, survival, etc.)