OpenLayers.org is an open source JavaScript web mapping library distinguished from other alternatives, like Leaflet or Google Maps APIs, because of its huge set of components. Having been on the scene for a long time, there is a large set of resources and documentations, and a wide and actively supportive community.
OpenLayers lets you easily develop “real” web maps without having to be a super experienced programmer or a GIS specialist. These examples provide an overview of the most popular vector and raster data formats currently being used to share spatial information on the Web. Put maps in your web projects, from simple base maps to interactive geo mash-ups, Google Maps, OpenStreetmap, GeoRSS, Panoramio, and your data. Starting with a lot of geo-information sources and a powerful library, the example apps explore how to manage maps inside web pages using basic third-party base maps to the overlay of re-projected vector data, going through styling thematic data and data interaction.