When you need to get your eyes examined, there are two different categories of medically trained professionals who can help you.  Both an optometrist and an ophthalmologist are referred to as an eye doctor, although their roles and training differ greatly.

An optometrist is trained to test your vision to see if you are nearsighted or farsighted. When you think of an eye doctor, this is probably who you think of, although an optometrist has a “Doctor of Optometry” degree rather than a medical doctor degree. An optometrist can give you a test with an eye chart, and then determine your prescription for contact lenses or glasses.  He or she will also be able to recognize symptoms of additional problems such as astigmatism, color blindness, glaucoma, or cataracts, and will refer you to other professionals as needed. When you visit an optometrist, you may need to have your pupils dilated; this helps the optometrist see more of the inside of your eye and make diagnoses accordingly. You should plan for this when you have your eye doctor visit; bring sunglasses so that afterwards the sunlight does not hurt your eyes.  Pupil dilation can also make it difficult to drive; you should arrange to have someone pick you up from your exam, or wait until your pupils shrink back to their normal size. This may seem inconvenient, but it is an important part of your eye doctor visit.

An ophthalmologist has been trained at medical school as a surgeon (in contrast to optometrists, who cannot perform surgery).  Eye problems other than vision correction are usually referred to an ophthalmologist.

You should also be aware that the professional who helps you at a glasses store is not an eye doctor; this person is an optician, trained to fulfill lens prescriptions from eye doctors.  An optician can help you find the right style of glasses for your face, and can ensure that you are given the right kind of lenses, but an optician is not an eye doctor who can make diagnoses.