Supposing you didn't see the news yesterday, there's word from Sony today that the PSN (PlayStation Network) will be down today starting at 9:15 AM PST until 3:15 PM PST. This is a standard amount of down-time and will affect the PlayStation Store, PlayStation Home community, or any of the account-management settings. This 6-hour period will be spent tuning up and maintaining the PlayStation Network and Sony has assured us the world will continue spinning while the downtime commences.
This downtime should have no effect on PlayStation users' ability to play online multiplayer games, supposing they've logged on to the PlayStation Network or have played a multiplayer online game in the past several days. Those attempting to access any PlayStation Network services during the maintenance period will be treated to a lovely notification which suggests they chill out for a short period.
Users should also be able to access apps during this maintenance period, supposing also that they'd signed in in the days before this scheduled maintenance period began. To clarify, this maintenance period is taking place on Wednesday, September 25th, 2013 - pay no attention to the notifications from past months and years (even though they're all essentially the same).
Those of you wondering what you should do with your life during this period of time should consider a brisk walk outside, some alone time with an offline game, or a browse through SlashGear's PlayStation archive. You'll also be happy to know that by the time you read this message, only three hours will remain of the break - good times!