Some privilege Qn

Im still not clear the differences b/w Comment, Moderate, Edit, Add
privilege levels.

My confusions are:

1- Access_Comment for articles = role group is allowed to add comments to
articles
BUT what about Access_Comment for Messages module?

2- what does Access_Moderate for articles (or any other modules eg
Comments, Messages etc) do ?

3- Access_Edit for articles module implies the ability to edit articles?
ie ATM on my site, only Admin can edit articles...

4- If I set Access_Add for Comments module, is that the same as setting
Access_Comment for articles too ?

Thank you.

Banzai "the newbie"- still trying to get a grip of the basics in Xaraya.