Trade is an integral part of diplomacy. When you select a player in a game there will be an option to trade either technology or credits.
Trading technology with another player has a transaction fee. For each trade involving technology, the system imposes a fee depending on the level of technology and the game's settings, which the gifting player pays when they send the technology. In a standard game the transfer cost of technology is $15 per tech level, so trading a level 2 technology would cost $30 and a level 3 technology would cost $45.
Alongside technology you can also send credits to another player. To send credits to a player simply input how much you want to give to the player in the text box at the bottom.
To keep track of trade there is a ledger. It details how much a player owes. Players have the option to repay their debt or the creditor can forgive all debt.
Depending on game settings a player may have to be in scanning distance with another player in order to trade with them.