There are 3 main type of sealers - topical (surface) sealers, penetrating sealers and impregnating sealers.
Topical Sealers

Topical sealers generally are water based acrylic or oil based polyurethane. Like wood varnish, they form a coating over the surface and tend to be cheaper than penetrating sealers but have disadvantages. Topical sealers are like a film over a floor. It looks great for a while.

Topical sealers are likely to noticeably change the appearance and slip resistance of the surface, lock in moisture and wear relatively quickly as they collect abrasive materials in the soft coating. When they wear through, the entire surface has to be stripped and resealed.

Penetrating Sealers

Penetrating sealers are generally longer lasting than topical sealers, but vary in performance. Many penetrating sealers use fluorocarbons (e.g. Teflon) or silicone, can work well and are somewhat breathable. Penetrating sealers still have a limited lifespan in comparison with the surfaces they are meant to protect.

Impregnating Sealers

Impregnating sealers are generally the longest lasting, fully breathable and allow water vapor to escape and provide the best protection without substantially altering the natural look or slip resistance of the surface.