Web Map Services allow you to display air photos and other maps on your computer without having to download all the data. The air photos and data are stored and maintained by the organization providing the service. The service only has to send a picture made from the small part of the dataset that you are looking at, as you need it. This efficiency means that the WMS display is generally very fast.
Using a WMS means:
You can use a WMS with free software such as Google Earth and ArcGIS Explorer, and with many different GIS software packages.