Roll-top tubs are large (normally cast-iron) bathtubs with a rounded edge. Usually free-standing, they recall images of luxuriant bathing in the Victorian era.
For the true Victorian experience, you should opt for a cast-iron tub, but these are quite expensive and not absolutely necessary for the look of your bathroom. Thankfully, acrylic roll-top bathtubs are also available for significantly less money.
You also need to consider the shape and size of your bathtub. If anyone tall will be using the tub, you should aim for a longer model — such as 61 or 68 inches. If you want to fit two people into the tub, consider going for a double-ended model so that both users can sit comfortably against a sloped end. These models will also have the faucets on one side instead of on the bottom end.