Why is bootstrap with CDN so powerful

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CSS

bootstrap

Why is bootstrap with CDN so powerful?

<link href="https://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/3.3.1/css/bootstrap.css" rel="stylesheet">    
    <script type="text/javascript" src="https://cdnjs.cloudflare.com/ajax/libs/jquery/2.1.1/jquery.js"></script>
    <script type="text/javascript" src="https://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/3.3.1/js/bootstrap.min.js"></script>

I can get bootstrap.css to offset and add text in shaded areas using classes in HTML.
Is it that you can use 2 CSS commands in one class?
What other important things can bootstrap do?
What does CDN - Control Delivery Network do?

What I have tried:

I tried listing all of bootstarap.css. It was too long, I need the highlights.