This is the question that I've been asked about a few times recently. ID3 tags, if you didn't already know, give you additional information on MP3 files such as, artist, song title, album, track number, etc. This information is embedded within the audio file in a special container as metadata (any media file can have meta information). The usual way to access and edit this information is via your software player, such as, iTunes or Winamp. But what if you need to edit hundreds of tracks? Using your software player to manually edit each track will quickly become tedious and have you pulling your hair out in no time.
TigoTago is a freeware program for Windows that lets you edit multiple audio files in the style of a spreadsheet. For example, if you need to fill in album track numbers for a series of files then TigoTago has a tool that automatically increments each track number. Another great feature of TigoTago is it can rename your files on your hard drive by using tag information. TigoTago also uses online CDDB resources to auto-fill complete albums; this is one of its biggest timesaving features.
For more information, read the TigoTago tutorial on editing ID3 tags.