There are three main types of sealers: topical, penetrating and integral.
Topical sealers are placed on the surface of the concrete and form a protective layer. These are good choices for both indoor and outdoor applications. Penetrating sealers actually penetrate the concrete and form a chemical barrier that shields against moisture. Integral sealers are added to the concrete mix before it leaves the plant. With integral sealers, you receive the same benefits of a topical sealer, but the sealing will start immediately.