Spend enough time around PC builders and you'll learn that the direction a case or CPU cooler fan blows matters. That's because fans are often oriented to pull cool air into the computer (known as intake) or to expel warm air out of the case (exhaust). Incorrectly positioning fans can severely impede airflow, leading to high temperatures and decreased cooling efficiency.
Fortunately, most fans have built-in indicators to help you determine which way they're blowing. Look closely at the casing of a fan to see if it has an arrow printed on it or other markings that indicate which way it's meant to blow. If it doesn't have an arrow or you're evaluating a CPU fan without its casing, the best thing to do is look at the blades.
The shape of the fan blades will give you a pretty good idea which way it's blowing. If the blades curve away from you, it's an intake fan and will be pulling in cooler air. If the blades curve toward you, it's an exhaust fan and will be pushing warm air out of the case.
Regardless of the orientation of a fan, it's important to keep them clean and dust free. This will improve their performance and extend their lifespan. Every six months or so, open up the case and blow the fans out with compressed air to remove any accumulated dust. And if you live in an especially dusty environment, you may need to do this more frequently.