Conjunctivitis is an inflammation of the outer surfaces of the eye and the inner surfaces of the eyelid. It's almost always due to either an infection or allergy. More rarely, it can be caused by exposure to a chemical.
When caused by a virus, the condition is usually referred to as pink eye, and it is characterized by a distinctive red colour, particularly where the eyeball meets the inner part of the eyelid. Bacterial conjuncitivis is often accompanied by a yellowish discharge that can harden on the eyelashes.