A file-format is the specific way of ordering and grouping of information in a (computer) file.
File-formats can explicitly defined in standards or documentation, or implicitly defined by acceptability to some processing program.
File-formats of explicitly defined standards can often evolve based on version numbers.
For ease of recognition file formats often have a unique sequence of first few bytes (the header). File-formats like EPUB, which start with the header of the ZIP container format the are build upon, have such recognisable 'header' in the first file in the container (called mimetype)
More detailed information can be found on wikipedia
