What is the difference between just being poor sighted and having a catract?

Glasses do not help. Poor sighted improves with glasses. Cataract occur from change in lens proteins which cause whitening/clouding of clear lens composition: can be due to an infection or inflammation from birth (ie, rubella), old age/genetics, steroid use, trauma, poor diet; smoking & Diabetes; vision decrease from cataracts do not improve with glasses. More info: eyedoc2020@blogspot.com.
Correctability. Cataracts mostly occur in eyes that are normal otherwise and removal of the cataract restores good vision. Poor sighted refers to uncorrectable vision; such a person will see badly no matter what is done.