Hi Stan, welcome to the Cameralabs forums!
Thomas is right, when you reduce the resolution, you capture fewer pixels, and therefore less detail.
Where it gets interesting though is if the camera can now exploit these 'unused' pixels to gather more light and increase sensitivity / reduce noise.
This technique, known as pixel binning is explained here:
As you'll read though, you'll probably need to drop the resolution down to a much lower level before a camera can start binning effectively.
Ultimately though take some test shots and see what you think.