Rollerball pens are similar to ballpoint pens in that they both operate by a tiny ball distributing ink onto the paper, but the ink in a rollerball pen is water based, like you would get in a fountain pen. This means that a rollerball pen needs a cap, to stop the ink from drying out, and also it means that the writing style is much more fluid than a ballpoint, while being much easier to maintain than a fountain pen.
Rollerball pens are a half way compromise between the two, taking the ease of use of a ballpoint, and combining it with the elegant writing style of a fountain pen.
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