Diplomacy

From the Encarta:‎

The world is full of Nations, each trying to expand and grow and protect their Citizens. You will inevitably run into other Nations as you explore and expand your Borders. You may try to ignore them, but you will have to interact with each other to thrive.

Trade

From the Encarta:‎

Trade Agreements are good ways to exchange Resources and Goods between Nations. To establish a Trade Agreement, you need both Open Borders and a Neutral or higher Relationship.

Once you have established an Agreement, you can select the "View Trade Agreement" button in the Leader's Diplomacy Screen. Choose what you want from the other Nation's inventory, and your Merchants will go and will start bringing it to you every few turns.

The number of items you receive from Trade Agreements is limited by the number of Merchants in your Nation. Most Nations will start with 2 Merchants, but can get more through various unlocks such as Improvements later in the game. You can also make Trade Routes go faster by joining your Nations with Roads.

Trade Routes may be broken by War or merely because a participant opts out.