Mapnik is an open-source toolkit for developing mapping applications. It can be used for both desktop map design and web development, and provides algorithms and patterns for spatial data access and visualization.
Mapnik itself is not a Mapbox product, but Mapbox engineers have contributed to the project and it is used in several Mapbox products. For example, Mapnik is used to render tiles from classic styles when they are requested via the Mapbox Raster Tiles API. Mapnik was also used to render maps within Mapbox Studio Classic.