Server maintenance check

I was wondering what happens during a server check for websites and any online programs. I was just on Chase banking online and they were on a maintenance check so i couldnt pay my bills >=O. Then i went to youtube, above it stated that they too, was undergoing a server check as well. Yet when i view videos everything seems just fine. So im wondering whats going how can some sites work and others don't... I should worry about my bills first though.

- Kevin Ben Lee