The Ladder and Casual screens provides a way for you to play against other people of similar skill.
You may select a range of 2-4 players. If you want a specific number of players, just set both the minimum and maximum to that number.
When you press "Start Search" the system adds you to the matchmaking queue. When enough players with the same criteria are available, your new game is created. The system tries to find players who are of equal skill. The longer you wait, the wider range it allows for the skill matching.
The system will pool all the sets and Environments owned by all the players in the game and select from that pool. This means, even if you don't own a particular set, if one of the other players in the game does, Things in that set will be available to be included in the game.
With Environments, there is about a 20% chance of one being included in the game. Additional Environments may be included, but the chance gets smaller each time another is added to the game.
Chat is disabled during Ladder and Casual play. However, at the end of the game, you may invite opponents to be your friend. If they accept, you can create Friendly games with them which allows you to chat.