Open Layers is a pure javascript + AJAX library for displaying and editing geospatial map data. It supports accessing OGC compliant WMS as well as WFS services and also GeoRss, GML and SVG standards. It also supports proprietary Web Mapping Services such as Google Maps, DMSolutions Kmap Framework, Microsoft Virtual Earth, KML, and tab delimeted text files.
It has preliminary support as a WFS-T client. This means you can edit data using it via a server that supports WFS-T such as Geoserver.
It also has support for reading and editing Well-Known-Text Format (WKT) which means you can use it in conjunction with a spatial database such as PostGIS, MySQL, Oracle Spatial, DB 2 Spatial Blade that can directly output and consume WKT geometries.
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The OpenLayers API documentation can be found here. OpenLayers Class Hierarchy
