You've probably heard of probiotics, especially if you spend any time in the yogurt aisles of supermarkets, but what are they exactly? Are they good for your heath? And should you join the millions of Americans who already take them?

To answer those questions, here's a primer. First, most probiotics are live cultures of microorganisms. These bacteria are similar or just like those that are naturally found in your gut and help your body function properly.

You can find probiotics not only in yogurt, but in other fermented dairy products (such as kefir and aged cheeses), some nondairy foods (including kimchi, sauerkraut, miso and tempeh) and dietary supplements.

And what about their health benefits? There's evidence that some probiotics may help prevent diarrhea caused by infections or antibiotics. Some may also ease the symptoms of irritable bowel syndrome or lactose intolerance.

The U.S. Food and Drug Administration hasn't approved any health claims for probiotics and more research is needed to be certain of how they may—or may not—boost health.

One caution: Be sure to check with your doctor before taking any probiotic supplement. They have a good safety record in generally healthy people, but they may cause severe side effects in people with serious health problems.

Sources: Academy of Nutrition and Dietetics; National Institutes of Health