There are two main types of ink that are used for textile printing. Water-based ink and Plastisol. Plastisols are the most commonly used inks for printing designs onto garments and are particularly useful for printing opaque graphics on dark fabrics.
Plastisol inks are not water-soluble. The ink is composed of PVC particles suspended in a plasticizing emulsion, and will not dry if left on the screen for extended periods. Because of the convenience of not needing to wash a screen after printing, plastisol inks can be used without a source of running water. Plastisol inks are recommended for printing on colored fabric. On lighter fabric, plastisol is extremely opaque and can retain a bright image for many years with proper care.
Plastisol inks will not dry but must be cured. Curing can be done with a flash dryer, or more inexpensively, a home oven. Most plastisols need to reach a temperature of about 180 degrees Celsius (350 Fahrenheit) for full curing. Plastisol tends to sit atop the threads instead of soaking into them, giving the print a raised, plasticized texture. Other inks can produce a softer feel.
THE BOTTOM LINE: For the average customer it comes down to cost and feels. Most t-shirts are printed with plastisol. It is more cost-effective but you will be able to feel the ink on the garment. Waterbase and Discharge on the other hand are softer to the touch and sometimes you can't feel the print at all. But because waterbased and discharge are more difficult to use, printing with these inks will cost more than plastisol.