While there are legacy and future standards for ID3 tags, the most popular version implemented today is ID3 version 2.3. A follow on version, 2.4, has not achieved popular status due to some disagreements on some of the revisions and the tremendous inertia present in the software and hardware marketplace. Full details of the ID3 standard can be found at http://www.id3.org/
ID3TagEd generally follows the 2.3 standard. A copy of the standard, taken from the id3 web site, may be viewed here. A new window will open.