Begging the question is an example of a fallacy of presumption, also known as a circular argument: The conclusion appears at the beginning and the end of the argument. A is true because A is true.
A valid argument in support of a claim will offer evidence or reasons independent of the claim.
"The law says you should drive on the right side of the road, and the law is the law."
When someone is questioning this statement, they are questioning the law. If we say, "because that is the law," we are begging the question. We are assuming the validity of what the other person is questioning.
The most simple form of begging the question: A is true because A is true.
Circular reasoning can also be a bit longer: