HTC Incredible :: Format Phone Storage Vs Clear Storage

So if I do a Factory Reset on the phone it still has some of the old photos/videos I took, which leads me to believe it doesn't fully clear the device of everything, just the settings that I changed are returned to their factory defaults (duh!).I was curious if I do the Clear Storage option, which means turning off the device and holding one of the volume toggles while holding the power button - would that mean that the entire Android OS would be removed, or does it mean it does a format of the phones entire partition and re-installs Android (similar to a reformat/recovery partition on a computer)? In which case I would need to reactivate the phone doing the simple *228.