In a few words, MP3 Diags is a GUI-based application that allows end-users to identify issues with their MP3 files, fix some of the issues and make other changes, like adding track information. It also lets you "look inside" an MP3 file.
Unlike some programs that are designed to deal with a single Topic (like fixing VBR headers or adding cover art), MP3 Diags is a one stop solution that identifies more than 50 different issues in MP3 files and provides the means to fix many of them (well, not everything is fixable; you can't make a 64kbps file sound like a 256kbps one.) Some of the more important issues that are found: Broken tags / headers / audio duplicate tags / headers incorrect placement of tags / headers (ID3V2, ID3V1, LAME, Xing, ...) low quality audio missing VBR header missing track info / cover art broken track info / cover art missing normalization data character encoding issues (for languages other than English).
Some of the fixes and changes that MP3 Diags can do:
Adding / fixing track info, including album cover; information can be retrieved from several sources: Internet, clipboard, file name, local files, and (obviously) keyboard Correcting files that show incorrect song duration Correcting files in which the player cannot seek correctly Converting characters for non-English names Adding composer name to the artist field, for players that don't handle the composer field Renaming files based on their fields Changing word case for track info.