It’s not clear exactly why the service is currently unavailable, and it looks like only the digital download store is affected for now. Steam’s site currently reads:
An error occurred while processing your request.
The desktop app loads fine and allows you to access your library, as well as download and launch games. However, the store is unavailable both here and via browsers.
We’ve contacted Steam to find out more and will update this post as soon as we hear back.
Update: Steam’s store is back online now after roughly an hour of unscheduled downtime. Back to your battlestations, everyone!