Projections/Spatial reference systems
Data available on this site is in one of these two projections/spatial reference systems.
WGS84 (EPSG 4326)
OpenStreetMap uses the WGS84 spatial reference system used by the Global Positioning System (GPS). It uses geographic coordinates between -180° and 180° longitude and -90° and 90° latitude. So this is the "native" OSM format.
This is the right choice for you if you need geographical coordinates or want to transform the coordinates into some other spatial reference system or projection.
Mercator (EPSG 3857)
Most tiled web maps (such as the standard OSM maps and Google Maps) use this Mercator projection.
The map area of such maps is a square with x and y coordiates both between -20,037,508.34 and 20,037,508.34 meters. As a result data north of about 85.1° and south of about -85.1° latitude can not be shown and has been cut off.
The correct EPSG code for this data is 3857 and this is what the data files show. Before this code was allocated, other codes such as 900913 were used. If your software does not understand the 3857 code you might need to upgrade. See this page for all the details.
This is the right choice for you if you are creating tiled web maps.