A rollerball pen utilizes less viscous water-based ink or gelled ink. Often, but not always, a rollerball pen will have a cap that covers the point and can be removed. Rollerball refills can be replaced when the ink is gone.
Ballpoint pens are the most used writing instrument in the world. The viscous ink is - usually from paste - is dispensed over a metal ball at its point. Generally, they do not have a cap and the ballpoint refill is exposed through twisting or clicking of the cap.