CartoCSS is a CSS-like language used in TileMill and Mapbox Studio Classic to style features.
CartoCSS is the language for map design used by Mapbox Studio Classic (and the deprecated TileMill. It is similiar in syntax to CSS, but builds upon it with specific abilities to filter map data and by providing things like variables.
CartoCSS, also known as Carto, targets the Mapnik renderer and is able to generate Mapnik XML.
CartoCSS is an evolution of the Cascadenik idea and language, with an emphasis on speed and flexibility. For more information on CartoCSS, see the Mapbox Github repository for Carto.