High total or LDL cholesterol tends to promote hardening of the arteries and thus raises your risk of heart attack, stroke, and artery disease. Smoking, hypertension, and diabetes magnify the increase in risk. Weight loss and a diet that is low in animal fats help reduce cholesterol.
HDL cholesterol ('good cholesterol') is raised by exercise. A high HDL cholesterol represents cholesterol being removed from arteries and lowers your risk of the diseases above.
There is more information about cholesterol available on the Centers for Disease Control and from the American Heart Association. Here is another site with sensible diet tips.