Steam Guard is a security measure that users of Steam have been able to use for over a year now. When activated, it will send a code to your email when your Steam log-in is being used on a new computer. This is meant to stop those who otherwise might gain access to your account. So far, it has not been enforced in any way although many have activated it anyway. If you're a fan of using Steam Trading then be aware of the upcoming requirement.
Steam Trading was added after people expressed a need for something like it. What it does is allows two people to trade games at once instead of relying on the other to gift them something back. Using Trading currently reveals that, starting on December 12th, it will be compulsory for Steam Trade users to have Steam Guard activated for their account.
The message also states that accounts will have to have Steam Guard enabled for fifteen days before being allowed to trade items. This new rule only applies from December 12th onward. Although most of us already have Steam Guard enabled, there are certainly those out there who have opted out of it until now. The push for security is probably being done in preparation for the next big Steam sale. So, if you expect to be trading games around then be sure to activate the extra level of account security.
Do you use Steam Guard? Why or why not?