Question: What is the difference between a Ballpoint pens/refills and Rollerball pens/refills?
Answer: Ballpoint pen refills use a thick oil based ink where the ball in the pen nib pushes the ink on to the paper. Ballpoint pens have a thick ink that does not evaporate easily. A cap is not necessary. Ballpoint ink is less likely to bleed through the paper.
A Rollerball pen is more related to the fountain pen as it uses a water based ink. The Rollerball pen and refill was first introduced to fit the demands of loyalists of fountain pens that missed the use of a wetter ink. Rollerball ink spreads on paper widely and goes inside the paper a little deeper than oil-based ink of a ballpoint pen.
Since the water based ink of a Rollerball pen has a higher rate of drying out, having a cap is absolutely necessary for this pen to cover the pen nib when not in use.
Most Ballpoint and Rollerball refills are offered in fine and medium size, some brands offer broad.