Conjunctivitis (commonly called pink eye): Inflammation of the conjunctiva caused by allergies or infections (viral or bacterial); blood vessels in the conjunctiva become engorged, and the normally white part of the eye looks red. Other symptoms usually include itchiness and discharge.
Corneal abrasions: A scratch on the cornea is called an abrasion. It can be very painful. The cornea is vulnerable to injuries from fingers, errant tree branches, or tennis balls. With antibiotic drops and monitoring by your doctor, corneal abrasions tend to get better without further problems.
Corneal infections (called keratitis): Inflamed or infected cornea sometimes caused by bacterial or viral infections; these infections are often associated with wearing contact lenses overnight or wearing lenses that haven't been properly cleaned and disinfected.
Foreign bodies: Something in the eye -- a bit of dirt, plant debris, or a fragment of a contact lens; these are usually just irritating, and tears or water rinse them out. If not removed, foreign bodies can cause corneal abrasions.
Glaucoma: Eye condition that usually has no early symptoms. In the case of acute angle-closure glaucoma, though, pressure inside the eye rises suddenly. Symptoms include severe eye pain, nausea and vomiting, headache, and worsening vision. These symptoms are an emergency and need immediate treatment to prevent blindness.