Ophthalmology is the branch of medicine that deals with the anatomy, physiology and diseases of the eye. An ophthalmologist is a specialist in medical and surgical eye problems. Ophthalmology includes sub-specialities which deal either with certain diseases or diseases of certain parts of the eye.
Some of them are:
Anterior Segment Surgery
Retinal Ophthalmology - emphasis on LASER treatment of the retina and actual retinal surgery
Cataracts - not usually considered a subspecialty per se, since most general ophthalmologists perform cataract surgery
Cornea, Ocular Surface and External Disease
Medical Retina deals with treatment of retinal problems through non-surgical means
Oculoplastics & Orbit Surgery
Pediatric Ophthalmology/Strabismus (mis-alignment of the eyes)
Vitreo-retinal Surgery deals with surgical management of retinal and posterior segment diseases and disorders
Medical Retina and Vitreo-retinal Surgery sometimes together called posterior segment subspecialisation.