Web Mercator (EPSG:3857) is the projected coordinate system that underpins virtually every slippy web map — Google Maps, OpenStreetMap, Mapbox, MapLibre, Leaflet. It is a variant of the Mercator projection that treats the Earth as a sphere of radius 6,378,137 m (the WGS 84 semi-major axis) while taking coordinates from the WGS 84 ellipsoid, a deliberate simplification that makes tile math fast.
Why it matters
Web Mercator is conformal (it preserves local angles and shapes), which keeps north "up" and makes tiling at fixed power-of-two zoom levels trivial. That is exactly what you want for a basemap. The trade-off is severe area distortion: features near the poles are enormously inflated. Greenland looks roughly the size of Africa, though it is about 14 times smaller. The projection is also undefined beyond roughly ±85.06° latitude, which is why web maps clip at those parallels.
Concrete example
In Web Mercator, coordinates are in meters spanning roughly ±20,037,508 m on both axes. A point at WGS 84 lon/lat (2.3522, 48.8566) (Paris) becomes approximately (261,848, 6,250,566) in EPSG:3857. Convert with GDAL: gdalwarp -t_srs EPSG:3857 in.tif out.tif, or in PostGIS with ST_Transform(geom, 3857).
Common pitfall
Never run area, length, or buffer analysis in EPSG:3857. Because the scale factor grows with latitude, a "1 km" buffer drawn in Web Mercator units is only correct at the equator and is badly wrong elsewhere. Project to an equal-area or local UTM CRS first, compute, then reproject results back to 3857 only for display. Treat Web Mercator as a presentation layer, not an analysis projection.