Let's check both definition by Google dictionary:
rub (someone or something) hard so as to clean them, typically with a brush and water.
"he had to scrub the floor"
move one's hand or a cloth repeatedly back and forth on the surface of (something) with firm pressure.
"she rubbed her arm, where she had a large bruise"
In short, Scrub means 'esfregar' in a sense of cleaning something, the floor, the bathtub for example.
Rub, on the other hand, is just the act of moving something (such as your hand) back and forth along the surface of (something) while pressing.