Yay!!! Finally caught up!! (I think...)
Ozone's quick high level overview of ballasts vs drivers. I don't want to say they're the same, because they're not. But they both perform the same basic function in that they take your input electrical supply and convert it to what your light source needs to make it work. So in the US it's your typical 120V AC 60Hz power that it takes and converts to whatever the HPS/LED/Fluorescent bulb need. So very important that what it does output matches what the LED needs in the case of a driver. Haven't used HPS personally, but I assume there are different wattage of ballasts for different bulbs/fixtures and the driver to LED matching is the same. It's just that since LED's are more DIY, build from scratch, there's not the straight correlation like matching up fixtures with drivers and you have to read up and know which numbers to match to which to make it all work.
Started out to try and make it less confusing and think I may have even confused myself now.....
*mutters notes about electricity under his breath while wandering off to go have a dab*