TileMill is a tool for cartographers to quickly and easily design maps for the web using custom data. It is built on the powerful open-source map rendering library Mapnik - the same software OpenStreetMap and MapQuest use to make some of their maps. TileMill is not intended to be a general-purpose cartography tool, but rather focuses on streamlining and simplifying a narrow set of use cases. For anyone coming from a GIS or cartography background, the biggest assumption TileMill makes is the final projection - TileMill maps are always projected to “Web Mercator”. As the name suggests, this projection is popular with web mapping applications, thus maps created with TileMill can be displayed using Leaflet, the map APIs from Google and Apple, and many other projects. TileMill can export directly to a Mapbox account, or to your computer using the SQLite-based MBTiles file format, which allows for easy transfer or use in offline-capable applications. TileMill is a powerful design studio, but in order to use it effectively you will need to have several other types of applications installed alongside it: We cover some of the basics needed to use these programs with TileMill in this manual, but for full details you will need to refer to the documentation of each of these individual projects. Source.