Very basically:
Opaques - cover up what is underneath. Even reduced, which can give the appearance of translucency, enough layers will reach solid coverage. It will also get to the point it matches the colour in the bottle, and no matter how many layers are added will not get deeper.
Transparent - will allow the layer below to show through, and also interact with it. For example spraying yellow over blue, will become green so you can in effect mix colour directly on the surface ( a barrier coat will prevent this), giving lots of subtle blends and hues. Also the more layers you add the deeper the colours get, until eventually they would become black.
That's the basics, if Mark finds the link that's the proper low down.
To add my own perspective, when I paint motorcycles, I tend to do an underpainting with opaque (to cover the base) then use trans as when its cleared it pops more and looks cleaner and brighter, while opaques can tend to look almost chalky in comparison.