Rollerballs
The rollerball pen is in many ways similar to a standard ballpoint pen, as they have a ballpoint mechanism just like in a ballpoint pen. The difference between the two types of pen is in the ink. Rollerballs use water based liquid inks or gelled inks, unlike the inks in ballpoints, which are viscous and oil-based.
The liquid ink type of rollerball (the other type is the gel ink, which is worthy of considering on its own, as it has its own advantages) also has similarities to a fountain pen. In fact, they are intended to combine features of both ballpoint and fountain pens, giving the convenience of a ballpoint with the smooth effect of a fountain pen. The tips of rollerballs also tend to be finer than ballpoints, and much finer than fountain pens. Different tip sizes are available, of course.
I personally find rollerballs much more enjoyable to write with than ballpoints, as the writing is smoother – sometimes ballpoints seem to “scratch” the paper, especially as it’s necessary to press harder with a ballpoint – and the ink flows better and doesn’t tend to leave the “gaps” that ballpoint pens do (unless the ink has run out, of course), lacking the occasional jerkiness of ballpoints and even gel pens. They tend to be more comfortable and faster to write with than ballpoints, and can be found in a much greater range of colours than the ballpoint’s standard black, blue and red.
Rollerball pens are available as both disposable and reusable (some disposable pens could be reused, if new ink was bought for them, but this generally is not worth considering – the ink is usually more expensive than the pen was in the first place!). Disposable rollerballs are usually more expensive than disposable ballpoints, though – disposable ballpoints are truly disposable. Reusable rollerballs do tend to be better – you get what you pay for.
Rollerballs (and therefore gel ink pens as well, but generally to a lesser extent) are my own preferred everyday pen type for writing with and carrying in my pocket. Which type you prefer is up to you, and what size of tip you prefer. I usually write with a medium or fine tipped rollerball (typically 0.5 mm or 0.1 mm).