Several of Apple's online services went down Thursday afternoon, including the App Store, iTunes Store, iBooks Store, iTunes in the Cloud, iTunes Match, iTunes Radio, the Mac App Store, and OS X Software Update. It was the second outage in two weeks, but this one was brief, lasting just over 90 minutes.
Apple's own System Status page reported the services as "unavailable for some users." I was personally able to download updates through OS X Software Update and purchase and download a new app through the App Store, meaning the outage is not universal.
Apple's System Status as of 4:16 PM PDT
All Services Back Up By the Time This Article Was Completed
On March 11th, several of Apple's services went offline, including many of the same services that are currently down. At the time, Apple blamed the outage on an internal DNS issue that was resolved after a few hours. The company has not yet commented on this outage.
Note: As I was about to hit submit on this article, I refreshed Apple's System Status for the 50th time while writing it and found all services back online. I recouched the article accordingly.