OPF files are XML-based files in the Open Packaging Format Standard. The OPF standard is part of the EPUB specification and stores metadata such as title, language, categories and author of an ebook. In addition, images and the navigation structure are referenced in an OPF file.
An EPUB publication usually contains a file called Content.Opf and an associated toc.Ncx file with the table of contents. The actual content pages are html files without which an OPF file can not be meaningfully used.