The PlayStation Network will undergo scheduled maintenance today from 8AM until 11PM Pacific. The maintenance affects both North America and Europe. So, that's 11AM until 2AM Eastern and 16:00 until 07:00 Friday in the UK.
According to Sony, as long as you have signed in to PSN within five days prior to the maintenance, you will be able to play games online and use apps. If you haven't, you may receive a message saying "Playstation Network is undergoing maintenance."
The PlayStation Twitter will give the "all clear" when full functionality is restored.