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:
250 = Two hundred and fifty
Two hundreds and fifty
7,812 = Seven thousand, eight hundred and twelve
Seven thousands, eight hundreds and twelve
$10,000,000 = Ten million dollars
Ten millions of dollars