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

Best Valentines Day Gifts

Engraved Tagua Nut

Shopping for someone who loves exotic things? This engraved Tagua nut is unlike any other gift weve seen. This nut from the forests of Ecuador is engraved with a romantic message. This natural tagua is nestled in a box containing an information card telling the story of tagua.