Tagging for Dummies

[RichardFlack]

Im a bit confused by the "Compilations", "Various Artists", and 'Artist', 'Album Artist' and 'Contributing Artists'.

What is the definition of a Compilation?

I guess the real question is are there forma definitions as to how these terms are used and thus when they should be set by the tagging server. I think my problem is that my tags have probably been compiled over the various from various sources.
I have a sneaking suspicion there may not be defined specs as such; but are there at least general conventions.
Is there an easy way to 'normalise' a library other than goiing throughand doing a bunch of manual batch changes?

Im also seeing anomolies as to how things are listed in desktop iTunes, iOS Music and foobar, but I need to look at that more to better describe whats happening.

Addendum:

What is the best source for tags ... MusicBrainz Picard?