In an attempt to prevent Malware from making trades on behalf of the user, Valve announced that a Captcha system has been added to Steam's trading process. This Captcha system will ask the user if he/she is a robot or not, and only after answering the question will the trade proceed.
"We're updating trading to include a captcha as part of confirmation process. This is to prevent malware on users' machines making trades on their behalf. We know it's a bit of a hassle, and we don't like making trading harder for users, but we do expect it to significantly help customers who are tricked into downloading and running malware from losing their items," said Valve.
"We've excluded a few of the existing third-party trading services from this requirement so they can continue to function. If you have any suggestions on the trading process please let us know. Thanks."
At the time of writing, several people have been experiencing some issues with this new system, with the most common one being the fact that trade offers are being reset.
Source: Steam Community