An aptitude test measures your ability to do a whole bunch of (often unrelated) things. For example, if you’re going to college, you’ll take the SAT. If you're NOT going to college, other aptitude tests exist...ones that test your ability to fix a car, paint a car, change a car's oil, etc.
Aptitude is simply your ability or attitude to be um...apt. Or good at something. You’ll find them in places of employment, and even in the military and they exist for the sake of efficiency. Score low on the Biology AP exam and well, you're not going to be a surgeon. At least not in the U.S. And that's not the end of the world...you'll find the area in which you are most apt to succeed and...go for that one instead.