The common black molly is the one shown above, but there are a number of different color, shape, and fin varieties that have been bred today; they include the dalmation molly (white with black speckles), the silver molly, and a squat-bodied form called the balloon molly (they all originated from the wild saifin molly, P. latipinna, and all can also be bred to be longfinned).
While it is true that most mollies prefer slightly alkaline water, and really are borderline brackish water fish which do best with a little sea salt, from my experience, they are also a highly adaptable and robust group. I have no mollies currently, but always found them to be very hardy when I kept them years ago. They are usually relatively peaceful fish, grazing on algae when they aren't filling up their greedy little bellies, but sometimes a large female will turn into a bully of smaller fish.
Like all livebearers, the molly is very easy to breed, and will continue to do so like rabbits if conditions are to their liking. See the platy profile page to learn more about breeding livebearers.