By sealing your concrete floor, you're preventing moisture from seeping in, over time. In a basement or garage, a sealant is especially important as it will help prevent mold from forming in your home. Moisture will also form cracks in your floor, which a good sealant will prevent.
There are two main types of concrete floor sealant: penetrating and film-forming. Film-forming sealants work similarly to paints, forming a protective film over the surface which repels moisture. Penetrating sealants seep into the pores of the concrete, and form a resistance to water and stains. In 2005, the Environmental Protection Agency (EPA) examined the two types of sealant. While its study aimed to gauge their effectiveness on wood surfaces, the results can be applied to concrete surfaces, too. The EPA concluded that penetrating sealants were more effective long-term, as they did not tend to peel and flake as film-forming sealants are apt to do. As for the look you're trying to create with your sealant, each type has a variety of colors and glossy finishes to choose from.
Prepare the Concrete
Once you've chosen your sealant, prepare your concrete for coating by cleaning it thoroughly. If you're sealing your driveway, apply a high-pressure water hose to clean the surface of all dirt and gravel. Indoors, clean your garage or basement floor thoroughly of all grease and oil stains. A clean surface is imperative for your sealant to adhere properly. Once your floor is ready, apply the primer. Primer and sealant are toxic, so wear protective gear, such as goggles and a respiratory mask, when coating the floor. Also, keep an electric fan running close by. To apply the primer, you'll need painting materials such as a roller and splash pan. Apply the primer to the concrete as you would apply paint. Once you have applied an even layer to the entire surface, allow the primer to dry for several hours.
Coat with Sealant
The second coat is the finishing layer. Apply it once the primer is completely dry, and in exactly the same manner. Inspect your floor closely for missed spots. Leave the second coat to dry overnight. In the morning, run over your floors with a quick sweep and coat of polish.