What are some conditions of the eyes?

There are many things that might go slightly wrong with your eyes.  Here are a few of them.

Vocab used in this post:

Retina: A thin layer of the back of the eyeball that transmits light signals to the brain.
Cornea: The visible part of the eye, that light is absorbed through to get into the eye.
Lens: The filter within your eye (behind the iris) that light is transmitted through, which focuses the light rays onto the retina in the back of the eye.

Nearsightedness (Myopia)

        This is a very common problem in people of all ages.  Usually it is a genetic condition, and is inherited from your parents and you have it from birth.  The term nearsightedness means that you can see better up close than you can far away, so objects that are far away appear fuzzy.  This happens because the eye is actually longer than average, so the light rays bouncing off of a far away object aren't focused on the retina, but focus in front of the retina, so you see the scene fuzzily.  Here is a diagram that illustrates the unusual length of a nearsighted eye:

Farsightedness (Hyperopia)

        Hyperopia is the exact opposite of myopia.  It means that you can see far away objects better than close ones, so close objects appear fuzzy and out of focus.  This condition can happen from birth (although your body can usually compensate for it when you're young), but the most common cases of hyperopia happen when people turn 40, and changes happen in the proteins in the lens of your eye.  Becoming farsighted as you age is called presbyopia. The muscles and proteins in your eyes weaken, and the lens focuses the light rays behind the retina, as if the eye were too short.  When you are farsighted from birth, it is because the eye actually is too short.  Here is a diagram to demonstrate hyperopia:

Astigmatism

        Astigmatism is when the eye isn't too long or too short, just not quite round.  It is shaped like a football, slightly longer one way than another. This condition causes both blurring and distortion of vision, making everything look oddly curved or bent.  To the right is an artist's interpretation of what a person with astigmatism might see when they look at a field.  People with astigmatism see this way because the lens focuses the rays of light in a lopsided way, so the focus is either in front of or behind the retina.  This diagram shows the shape of the cornea of the eye in a person with astigmatism:

        All of these conditions can be compensated for with one of three ways: glasses, contact lenses or laser refractive surgery.  The latter is the only permanent solution, but glasses and contacts are more common, especially among younger people.