What is a trade offer?
A trade offer is a prepackaged trade that you can send to a friend. You select the items to trade from your inventory and the items you’d like to receive from your friend's inventory, and send the trade offer. Your friend can then choose to accept, decline, or make a counter offer. Unlike regular Steam Trading, you don't need to both be online at the same time. Plus, trade offers can be sent and received using a web browser.