Demo

style.css
index.html

p { color: red; }

/* Will “win” over an element selector. */
.maroon { color: maroon; }

/* But IDs will supersede classes. */
#intro { color: blue; }

#warning { color: teal; }

<!DOCTYPE html>
<html>
	<head>
		<title>Specificity</title>
		<link href="style.css" rel="stylesheet">
	</head>
	<body>
		<p>This paragraph will be red, from the <em>element</em>.</p>
		<p class="maroon">This paragraph will be maroon, from the <em>class</em>.</p>
		<p>This paragraph will be red again, no <em>class</em>.</p>
		<h2 class="maroon" id="intro">This heading will be blue, from the <em>ID</em>, even though it has the <em>class</em> too</h2>
		<p id="warning">This paragraph will be teal, from the <em>ID</em>, beating the <em>element</em>.</p>
		<p class="maroon" style="color: gray;">This paragraph will be gray, from an <em>inline style</em>, which beats everything.</p>
	</body>
</html>