A concrete overlay is a thin (around ⅛ - ¼ inch) coating of concrete that is set on top of an existing concrete surface. A stamped concrete overlay differs in composition when you compare it to a regular concrete mix. So, whatever you do, dont try and pour regular concrete or cement over your existing concrete floor. It wont end well.
Instead, what we use is a mix of concrete and polymer resin. The polymers add a level of performance and adhesion that regular concrete does not display. Stamped concrete overlays are most often used to restore pool areas, walkways, hardscape features, and patios.
A stamped concrete overlay is essentially the same thing as stamped concrete, except it is poured over already-existing concrete; a concrete overlay is much less expensive than tearing out the entire concrete slab, disposing of the concrete, and pouring/contracting a company to bring in new material. Stamped concrete overlays can refresh and improve the look of tired, dull, and worn out concrete floors with much less hassle.
As an aside, concrete overlays cannot be applied to badly damaged, waterlogged, or cracked concrete surfaces. In the event of a heavily damaged concrete substrate, you may have to initiate the messy process of breaking up and tearing-out the concrete, then hauling it to the dump. Its time consuming and costly, but is necessary in certain cases. For minor damage, concrete overlays should be fine. Just make sure to talk to a concrete contractor to double check.
Generally, a homeowner will contract a concrete company to prepare the existing surface for a stamped concrete overlay, which means that a concrete grinder or a shot blasting machine will have to be used to grind down the surface to a bare and uniform level. This is heavy equipment that should be handled with extreme caution by certified professionals.
The goal of preparing the concrete is to make it look like plain, ordinary concrete again, so that itll be ready for the stamped concrete overlay process.