In the Beginners CSS section you learned some things about why CSS has the word cascading in it. Basically later styles over-rule earlier ones. So external styles are easily over-ridden by in-line styles.
However there is another rule to confuse that cascading. More specific selectors will take priority over less specific ones. Imagine this:
- in an external style sheet you set the color of paragraphs to white using a p selector
- in the actual HTML you use in-line styles to set the body color to black
A p selector is a more specific selector than the in-line style in body element. Therefore all paragraphs will be white. The more general body selector will still apply to heading elements and so on.