Coffee contains caffeine, which is a drug that stimulates the mechanism
of micturition (that is, it encourages urination).  It doesn't actually
increase the output of urine as a conventional diuretic would, in other
words--it just makes you want to urinate more.  At least this is what I
last recall reading about it (it was long considered a true diuretic,
but apparently it's not).

All I know is that the caffeine in Coca-Cola is deadly for me,
especially in cold weather.  Nothing else that I drink seems to have
such a strong diuretic effect (but I don't drink much else besides soda
pop, water, and milk).  I have to take care to avoid it if I'm not
sweating a lot.  I try to drink only caffeine-free Coke when I can get
it.

Yes caffeine increase urinary output, by multiple mechanisms.  It blocks
renal reabsorption; it is a vasopressor (partially via an increase in heart
rate).  It probably also stimulates micturition directly (as the other reply
suggests). Caffeine stimulates a number of other parasympathetic responses
also such as stomach acid secretion ... for many it also does a very good
job stimulating bowel movements (the coffee + cigarette manoeuver).

Depends. Regular=yes; decaf=no. It's the caffeine that causes diuresis.

Yes. Coffee contains water, which is  a diuetic. (Water makes you pee, so it
is a diuretic.)

Caffiene also increases the amount of urine you produce, so it also is a
diuretic. Caffiene is  not a strong diuretic for me (and it doesn't amke me
poop either). For others, it is a strong diuretic.

Jeff