Acrylic artist ink is actually extremely-fluid acrylic dye that uses fine pigments in an acrylic emulsion. Acrylic ink is not paint. Acrylic ink is pigment that is suspended in a thinner medium than acrylic paint. Paint is made from a base that is runny enough to be applied with a brush. Ink is made with paint dye that is added to water, thinning it so that it can flow through a calligraphy brush or pen tip, for example.
Acrylic inks are water resistant and non-clogging. They dry quickly and are permanent. These inks are excellent for achieving water color effects, or for working with airbrush.