Orthophotos are aerial photos that have been “orthorectifed,” or geometrically corrected so that the photo is projected in the same way a map is projected. They are raster data. The photo has been corrected for the tilt of the camera, topography, and lens/atmospheric distortion. Orthophotos generally fall into one of three rough categories depending on the color information they have: panchromatic, or black and white, three-band (red/green/blue), or four-band (RGB+near IR). Orthophotos come at a variety of resolutions, with more recent data generally having the highest resolution– generally under one foot, though in the recent past anything under one meter (or three feet) was considered pretty good.
Orthoimagery is often used for analysis and land use detection, especially if it has an infrared band. It is also very commonly used as a backdrop for vector or other datasets in a GIS visualization.
Much of the imagery for the United States seen in Google Earth is generally publicly available orthoimagery, downloadable from the United States Department of Agriculture (USDA) or from another Federal or State agency. There is a lot of orthoimagery available from the providers listed under Geospatial Databases, but you can also visit the National Digital Orthoimagery Program for a list of Federal and other data providers.