Ophthalmologist = Medical Doctor (MD) that specializes in the eyes. 4 years of medical school followed by an internship and residency like any other MD.

Optometrist = Doctor of Optometry. 4 year professional degree post Bachelors.

There's a lot of overlap between the two, but there are some things Ophthalmologists can do that Optometrists can't, like eye surgery and prescribing drugs.

Optician = Not a medical professional at all. Some states require an Associates Degree, many just require a certificate. Basically the person at a mall kiosk who helps you pick out eyeglass frames, puts the lenses that an Ophthalmologist or Optometrist prescribed into the frames, and fits them to your head.