Fountain pens use liquid ink. Ball points use printer's ink, which is naturally viscous goo and gets sheer-thinned by the ball to be transferred to the paper. It is generally also (unlike the liquid ink) not water-based.
Gel pens are mechanically like ball points but they're using ink that's pigment particles suspended in a water-based gel which is even more viscous than regular ballpoint ink. The result is more opaque colour that shows better on a wider range of surfaces and a larger range of ink features. (E.g., glitter pens.) Many people don't care about all that and are much happier with the quality of the writing experience; all the convenience of a ball-point and (very nearly) the low-pressure gliding writing experience of a fountain pen.
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Gel pens are mechanically like ball points but they're using ink that's pigment particles suspended in a water-based gel which is even more viscous than regular ballpoint ink. The result is more opaque colour that shows better on a wider range of surfaces and a larger range of ink features. (E.g., glitter pens.) Many people don't care about all that and are much happier with the quality of the writing experience; all the convenience of a ball-point and (very nearly) the low-pressure gliding writing experience of a fountain pen.