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Answer:

1. Most map layers are available in the Shapefile format introduced by Environmental  Systems Research Institute (ESRI).
    Here is a link to technical whitepaper for the Shapefile format. This information is in portable  document format (PDF). Reading it requires an Acrobat Reader plug-in for your web browser.  Here is a link to download Acrobat Reader.
    
2. We also make most of these map layers available in the Topological Vector Profile of the Spatial Data Transfer Standard (SDTS).
    
3. A limited number of map layers may be made available in an Arc/Info Export format.
    
4. Some map layers are actually images. These are distributed in GeoTIFF format.
    
5. Sometimes, what appears to be a map layer is not a map layer at all. That's true of the data tables that contain information about the occurrence of moths and butterflies in the  United States. These tables are released in the dBase file format. They can be used in any  software programs (such as spreadsheets and data base managers) that can read a generic dBase file. However, if you want to map this information (using a desktop mapping program, for instance), you must use these data tables in conjunction with the County Boundaries map layer since the species occurrence information is recorded by county.
    

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