How multiplayer works, and the different modes available

After selecting multiplayer you will be asked to choose a player (just as you would if you are playing single player). After you have selected or created your character you are brought to the next screen asking if you want to host or join a server.

Joining
To join a game, currently you need to either join someone that is on the same wifi /router as you (not over the internet), or they have to host the game over the internet using port forwarding, and give you their IP, port, and password (if required).

Once connected you will be loaded into the world the host is currently playing in.

Hosting
If you wish to host a game there a few things you need to consider. When playing on your mobile phone or tablet there are 3 different types of networks. Look at the following list to see what best describes your situation: 1. Local Wifi (Both players are on the same router) 2. Internet via router/wifi (host needs to have port forwarding set up- the next section explains how to do this) 3. Mobile Network (can’t host, but can join a game)

The most common/ easiest situation should be that two people have a phone/ tablet connected through the internet via wifi/ a router. You don't need to do anything special, host the game, click on the inventory button to see what your local IP most likely is in the bottom left corner(or on Android, you have to go to menu->system settings->wifi->click on your wireless routers name, and it will bring up a window with your Internal IP address at the bottom). Then just give someone on the same wifi your IP, password and port (if changed), and they should be able to join.

You can see additional information here: http://ackmi.com/2013/07/how-to-do-multiplayer-over-the-internet-currently/

Hosting a Server on your PC NEW 9/17/14
If you want to host a server on your PC that anyone can connect to, you can now download the stand alone server. It requires Java of course, so if you don't already have it, you'll need to install the Java runtime environment/ JRE: http://www.oracle.com/technetwork/java/javase/downloads/jre8-downloads-2133155.html

Once you have the JRE installed, you can download The Hinterlands server: http://ackmi.com/upload/the_hinterlands_server.jar

Save it to a folder you want to run the server from (I'll assume you put it in c:\the_hinterlands). Open a command prompt in windows (start menu->type 'cmd'-> then press enter). Then navigate to the folder you saved it in (cd \the_hinterlands). Finally you can run the server with the command 'java -jar the_hinterlands_server.jar', and you should be prompted to enter the information for your server. The server will attempt to ask your router to open an incoming port for people to connect to you, but if it can't, you'll need to manually open/forward the port on your router.

Port Forwarding
Port forwarding is used when you have a router and you wish to let users connect to your server through it. Keep in mind that port forwarding might cause security risks.

The information your router will ask you varies. If you don’t understand what information your router needs, or how to input the information portforward.com has a more indepth tutorial.

Once you locate your router's admin page, and find the Port Forwarding page; hit add new service or custom service. When you get a page asking to setup the new rule, it should prompt you on what you want to call it. You can name it whatever you want, but if you want something simple ‘hinterlands’ works. Then you want to look for "type". Select TCP. For the ports (internal and external), enter 54555 and a password (this is optional).

You should be ready to host a game now. Make sure when your friend is joining your game to give them the correct IP address, port number, and password (if needed).

Google Play Services Multiplayer
You can now also play with anyone over the internet using Google Play Services. It's a little easier than WIFI, but it's also more likely to have more lag. Just select either "Play Game", or "Play Creative", then "Multiplayer", chose or create a character, select "Google Play Services", then either "Invite Friends" to invite a friend to play with you, "See Invites" to see a friends invites, or "Random Game", to play with anyone else looking to play. Once you connect to someone, you have to vote for who will be the host/ be able to pick the world to play in. Then the host can pick the world while the other player waits, and the game will start.