<style type="text/css>
h1 { color : black }
em { color :red }
</style>

What's happen now? Take a look here:

<h1>This is a line black</h1>
<h1>...And this is another line...<em>but what?</em> This is a RED line!!</h1>

Ok, but I want black lines...I want ONLY text within <em> tags to turn red!

So, how you can see, ALL lines which contain <em> tags will be red. To avoid this problem, you can use contextual selectors. So, in the above example, you could have written:

h1 em { color : red }

In fact, in this way, only text within <em> tags (within <h1> tags) will turn red:

<h1>This is a <em>simple</em>line to show you contextual selectors...</h1>