This is a serious question, not a troll or a trick, as I really don't
know the answer to this: what's the difference between primer and
"regular" paint?

First of all, some ground rules: I'm hoping to get answers from someone
who actually knows what they're talking about (any paint chemists out
there? or at least someone familiar with the inside of the paint
industry), rather than the usual speculation. Therefore, any answers
from "hallerb" or "ransley" will be politely ignored.

If someone forced me to explain the difference, my answer would be
something like, "Well, duuuh, I think primer has more solids or
something; or it's made to stick better. Yeah, that's the ticket--it's
stickier!" Obviously, no definitive understanding of really what primer
is and why we're 'spozed to use it.

Extra credit for answers to the following:

o Actual differences between oil-based primer (which I intuitively
consider to be better without really understanding why) and water-based
(latex, acrylic, etc.) primer.

o Are there any paints made today which can be considered "self-priming"
and which do not require primer?

The assumption is that we're talking about painting wood products here,
not speciality paints like for metal or other materials.

Your explanation of this matter, which is kind of like the eternal
debate over WD-40 (what it is and isn't and why it's no good for
anything) will be greatly appreciated.