Use hundred when there is a specific number, and use hundreds when you don't know or can't count how many. This rule also applies to thousand/thousands and million/millions.
This skeleton is thousands of years old.
This skeleton is three thousand years old.
When saying numbers, always use the singular form:
Fact
Dr. Samuel A. Mudd was the physician who set the leg of Lincoln's assassin John Wilkes Booth ... and whose shame created the expression for ignominy, "His name is Mudd." .. More >>