Concealer does exactly what its name suggests, conceals. As with pretty much every other make up product, concealers come in powder, liquid and cream formulations with varying degrees of pigmentation and shades as well as a multitude of packaging designs.
Concealers are generally more pigmented versions of foundations and more often than not has a more creamy consistency so as to apply the most colour and blend seamlessly into the skin. The goal is to hide imperfections yet having it still look natural.