The most basic fluid dynamics principle involved is Bernoulli's principle, which states that if the speed is high, the pressure is low, and vice versa. The equation of continuity can also be used

Blood pressure is so sensitive to cholestrol build up because if the blood has a smaller tube to go through, the blood will have to go faster in order to supply the same amounts of blood it did before.

If the blood has to go through a smaller tube, and therefore go faster, isn't there less pressure because it is flowing faster? So why do people say "High Blood Pressure"?

A buildup (and a reduction in the radius of an artery) would result in higher blood velocity. According to Bernouli's Principle: P1+v1=P2+v2 this change in velocity would result in higher blood pressure elsewhere in the arteries.