Ballpoint pens generally use a thicker oil based ink (in some cases almost waxy). When the ink is rolled across the paper with a ballpoint pen the ink tends to stay on the surface of the paper. There are always exceptions but usually ballpoint pens take more pressure to lay down the ink. Rollerball pens tend to have a more fluid ink (water based ink) that flows easier due to a lower viscosity. The ink in a rollerball pens tends to saturate the paper and not just lay on the surface as a ballpoint.