Firstly let me say that the vast majority of people we see have absolutely nothing more wrong with their eyes than some kind of focussing error.

The image on the left shows a perfectly normal eye.
(click to enlarge)
Some of the eye conditions we routinely look out for are:
Glaucoma
Glaucoma is a condition where the pressure inside the eye becomes too great and begins to damage the delicate nerve structures which if left untreated can lead eventually to tunnel vision.
We perform 3 tests to check for this:
1. Pressure
2. Optic nerve head
3 Visual fields
Diabetes
Diabetes can affect vision if it isn't monitored carefully, with this in mind all diabetics are now given an annual check at the hospital to look for weakness in the retinal vessels.
Cataract
Think of what happens when you boil an egg, the white swells (punch a hole in the shell and watch it pluther out all over the place) then goes from clear to opaque. The same effect happens in older eyes but much more slowly, early cataracts often start with the eye becoming more short sighted as the lens swells.
distance vision blurs but near vision stays good then as the cataract develops the overall vision gradually blurs.
These day's cataract removal is a very simple procedure

This is too big a subject for me to cover here but these sites:
http://www.rnib.org.uk/eyehealth/eyeconditions/Pages/eye_conditions.aspx
and
http://www.goodhope.org.uk/departments/eyedept/v
contain a wealth of information for further reading