Ink is a gel or solution containing a colorant, usually black by default, that it typically used for art, writing, or design. This is by using a quill, brush, or pen. The utensil is dipped into ink or an inkwell containing ink and than using it to make impressions onto surfaces such as paper, papyrus, clothe, wood, etc.
Ink in some form or another since human civilizations were formed. Inks can be made by soot, minerals, animal byproducts, and most famously cephalopods that use inking as a defense mechanism.