Trading allows players to securely swap items with another player for free. You can also offer in-game money through the trade window if you want to sweeten the deal.
All you have to do is use the trade command on someone and if they accept the trade request, put the items you'd like to offer in the left hand of the GUI and accept your side of the trade by clicking the red wool to turn in green. Then if the other player accepts the trade also, after 3 seconds the trade will be complete!
Trades can happen at any distance, anywhere, between anyone! Its a very simple tool to use and love.
/trade playername - sends a trade request with whoever player you sent to.
/trade accept playername - accepts pending trade request by specific playername.