Web Map Service (WMS) is a protocol to serve georeferenced map images.
The basic idea is that, given a bounding box and some other parameters, such as a layer name, the client makes an HTTP request to the WMS server, which computes and returns an image with all the data for the specified layers and within the specified bounding box.
In OpenLayers, when you add a WMS layer to your map, the
OpenLayers.Layer.WMS instance is provided with some parameters, such as resolutions and tile size. The WMS server computes the right number of tiles for each zoom level and divides the layer in that number of tiles.
This way, when you add a WMS layer to the map, there is not only one request to the server, but one by each tile that ...