Some apps, but not all, you can close "Red X" but not quit, and they continue to work on whatever task you gave it. I think in the case of Browsers, they simply open back up more quickly from just closed and not quit.

For instance, when I download a torrent of a new linux distro with Transmission, i can close it, but it continues to download and upload. Mail I can close, but it continues to monitor my mail.

I think that's really the only big thing there. You could use the Activity Monitor in your Utilities to see if leaving them closed but not quit is using up all your memory, cpu, or otherwise.

Brian