These two personalities are often mixed. Many people who are simply shy are too often accused of being rude when in fact they are timid. This is a cruel world sometimes and so many people do not make allowances for people who are not interactive with other people.
Shyness is a type of inwardness of someone. It is a person who does not feel confident around self-assured people. It is not a rudeness but rather a self-lacking assuredness.
Rudeness is totally different. It is a state of total disregard for other people and an over obvious demanding behaviour. It is an “I am” attitude.
So, the next time you see a person who seems to tuck away rather than talk don’t naturally presume they are rude but rather shy.