Formats contain references to Files and Filesets to group together one or more files into what is commonly refered to as a Media Format. Formats are related to the Asset that owns them.
Document formats typically have one file, and might be Microsoft Word, RTF, or plain text files.
Video formats are more complicated than document formats, and typically you might have more than one format for a particular video - such as wrapper and codec format. The video may also have multiple file associated to them with different formats.
Most image formats such as JPEG, PNG or TIFF are made of one file, but if the actual asset is a Video asset, the format could be of Image Sequence type and comprise of many files.