Layer Types
OpenLayers has several different types of layers. They can be broadly grouped into two categories: Raster, and Vector.
Raster
Raster layers are imagery layers.
WMS
Google layers are loaded using the Google Maps API. As such, they require that you include the Google Maps API Javascript in your page in order to use them.
Like all proprietary layers, the Google Layer has a sphericalMercator option. Turning this to true causes the layer to be treated in projected coordinates in the 'spherical mercator' projection. (For more information, see :ref: spherical_mercator.)
The Google Layers accept a 'type' option. This option controls the type of the map. This type option is one of the options available through the Google Maps API GMapType Constants.
Vector
Vector layers are in-browser features -- points, lines, and polygons -- typically loaded from XML files.
