GeoJSON

GeoJSON is an open geodata format (RFC 7946) in which geography is written as ordinary JSON: an array of features, each with a geometry and a set of properties. A human can read it in a text editor, any programming language parses it in one line, and practically all web cartography understands it — Leaflet, Mapbox GL, OpenLayers, D3.js — with no plugins or converters.

An OSM map is exported to GeoJSON when you need the features themselves rather than a picture: to attach a layer to a website, compute statistics for a district, filter the categories you need, load the data into PostGIS or run it through GeoPandas. It is also the most convenient intermediate format: from it, ogr2ogr or QGIS will make SHP, GeoPackage, KML — anything you like.

The output is a single UTF-8 file: one FeatureCollection gathering all the layers of the selected area. Every feature keeps its OpenStreetMap attributes (name, type, tags) and a service property _layer with the name of its source layer — one line of filtering takes the data apart again into roads, buildings, water, greenery and points of interest. Coordinates are WGS84 (EPSG:4326), as the specification requires; the recorded precision is six decimal places, about ten centimeters on the ground: the file drops into any GIS and any web framework with no reprojection.