If you were attempting to log onto Steam during the year end festivities, you may have noticed that the store had been shut down in its entirety in the early morning of Christmas Day. Many users were reporting, before the store was shut down, being able to see private information from other user's accounts. This information included the last digits of Steam Guard phone numbers, the last two digits of credit card numbers, purchase history, and email addresses. However, none of which would be enough to allow someone to log in or purchase through your account. So, worry not!
Steam wanted to clarify with their customers just what it was that happened, as not knowing whether information was compromised had a few people worried. Early on Christmas morning, Steam was targeted by a DoS attack which increased traffic to the site by 2000%. Due to an error in Steam's caching configuration, improper information was shown to some users for just under three hours on Christmas day. Realizing this, Steam pulled the chord and the store went down.
Fortunately, the error only allowed people to view information rather than gain access to accounts, so no action is required on your end if you are worried. Also, your private information only had the potential of being displayed if you viewed a page on Steam with your information on display (an account page, billing info, etc.) between 11:50 AM and 1:20 PM PST on December 25. In addition to this, Steam is slowly trying to identify users that were affected and is contacting them accordingly.