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
Los Angeles's full name is "El Pueblo de Nuestra Senora la Reina de los Angeles de Porciuncula" and can be abbreviated to 3.63% of its size, "L.A." .. More >>