If you use "hg remove b"
against a file with "A" status, which means it has been added but not commited, Mercurial will respond:
not removing b: file has been marked for add (use forget to undo)
This response is a very clear explication of the difference between remove and forget.
My understanding is that "hg forget"
is for undoing an added but not committed file so that it is not tracked by version control; while "hg remove"
is for taking out a committed file from version control.
This thread has a example for using hg remove
against files of 7 different types of status.