Ball point ink Non water based, high viscosity paste ink (like molasses), which does not thin down when you write with it. Ink transfers from a rotating ball like on a printing press.
Rollerball ink Water based, low viscosity ink (like water), which takes very little effort to get it to flow hence smooth writing. Ink floods the ball point and in effect you just drag a pool of ink around the paper as you write. Typically, rollerball ink is not permanent and will run when exposed to water.
Gel ink Water based ink which has a viscosity which can change. The ink typically has a high viscosity when the pen is not being used, but when the ball starts to rotate, the ink thins down to have a low viscosity similar to rollerball ink. Ink transfers from the ball similar to both ballpen and rollerball a cross between a puddle of ink and printing press transfer to paper.