if you a trying to get people to subscribe to your free newsletter, a conversion would be a sign up.
if you were promoting a forum, a conversion would be a visitor that began posting.
etc.
typo correction
[edited by: PatrickDeese at 5:02 pm (utc) on June 2, 2004]
So: ROI = (profit per sale * conversion) - gross ad budget.
Easy to work out, and it's the bottom line, but amazing how many people don't bother to work it out.