The term PEN FLUIDS is a collective term for the various INKS used in pens. INKS are liquids that are used to write or draw on paper, and they come in a variety of colors and consistencies. They are usually made from pigments, dyes, and other chemicals, and can be water-based, oil-based, or even gel-based. Different types of INKS are used for different purposes, such as calligraphy, drawing, and writing.