Official Overpass API: http://wiki.openstreetmap.org/wiki/Overpass_API/Overpass_QL
Official Overpass tool: http://overpass-turbo.eu/
Official map features: http://wiki.openstreetmap.org/wiki/Map_Features
Some tutorial that seems educational: http://blog-en.openalfa.com/how-to-query-openstreetmap-using-the-overpass-api
Mapbox OSM tutorial: https://www.mapbox.com/help/overpass-turbo/
Overpass NPM package: https://www.npmjs.com/package/query-overpass