Dave pretty much sums it up with the photos he's used.
I general, when you've completed detailing a cars finish is usually starts by being dull and flat without allot shine an appeal. Looking closely you'll also discover swirls, minor scratches, water drop etching, and just plain old dirty.
A full detail with the right products, machine, and skill brings back that new finish look by removing all of the above. Adding on top of this work a good sealer and wax the finish will glow a glossy wet and have depth to it, almost 3D.
Water is another dead giveaway but this "might" only mean that someone's applied a sealant or wax to the finish but again, a closer inspection will reveal whether or not the minor surface defects have been removed..