What is the difference between Bootstrap .container and .container-fluid classes?

Just downloaded 3.1 and found in the docs…

Turn any fixed-width grid layout into a full-width layout by changing your outermost .container to .container-fluid.

Looking in bootstrap.css, it appears that .container-fluid is identical to .container. Both have the same CSS, and every instance of .container-fluid is paired with .container, and all column classes are specified in percentages.

When twiddling with examples I could not see any difference, as everything seemed fluid.

As I’m new to Bootstrap, I assume I’m missing something. Could someone take a minute and enlighten me?