File with an .OPF extension contains metadata of an electronic publication, such as: author, title, language, book’s ID, display method or read order. It is a part of the EPUB archive stored in compliance with the Open Packaging Format, in a format based on XML.

.OPF format, being a part of the EPUB specification, contains also the second file type which holds a description of the text of an e-book – NCX, also based on XML.