(NPR) - If you follow the vast world of fermented grapes, you may have noticed an influx of so-called natural wines. I fell under their spell a few years ago. Apparently, I'm not alone. There's something of a natural wine cult blooming in shops, bars and restaurants around the U.S.

Natural wines can be mystifying: The first time you drink them, they may be off-putting and nose-wrinkling. Some (especially the whites and roses) can be darker than usual, a little fizzy, cloudy or with good-sized clumps of yeast floating about. They're often rough, which some people find charming. Others think they're unsophisticated.

This quirky style of wine is quietly trending. So I reached out to a few experts to compile a primer of sorts.

Defining Natural Wine

"This is the most interesting thing happening in wine right now," says Stephen Meuse. He's the senior wine buyer at Formaggio Kitchen, a specialty food shop in Cambridge, Mass. Meuse also blogs, contributes to America's Test Kitchen Radio and was a Boston Globe wine columnist for 15 years.

He says natural wine is hard to define. "In theory, it's taking fruit that is grown at least organically, then taking it into the cellar and adding nothing, while also not taking anything away."

But to really understand natural wine, Meuse says you first need to understand how most modern wine is made.

Natural Wine Vs. Modern Wine

Most winemakers use additives to create consistent outcomes. For instance, carefully chosen, cultured yeasts are routinely relied on for fermentation. This gives the vintners the ability to control the process and customize flavor profiles. Those raspberry/banana notes you can find in a Beaujolais Nouveau? Specific yeasts help produce them.

Modern winemakers also make generous use of sulfur dioxide to prevent oxidation and bacterial growth. It also helps wine keep its brightness and snap, Meuse says. By some accounts, sulfur dioxide has been used in winemaking since the 15th century.

As for the naturalist winemakers, "On one end, you have people trying to edge themselves toward fewer additives, no additives, organic fruit — but then [they] will rely on a little sulfur or added yeast to correct problems," Meuse says. "And then, way out on the extreme end, are people who farm organically and then insist on letting the wine completely take its own direction. They're OK with organic matter in the bottle. And their wines take on different flavors. ... tangy, cloudy, yeasty." In the most extreme examples, Meuse adds, "you get wines that can remind you more of cider than wine."

Natural, unfiltered wines are still alive and can exhibit yeasty, microbial action. It's fun to observe their character changing as it sits in your glass.

'Organic' Does Not Mean Natural

But not all wine that's labeled organic would be considered "natural," says Philippe Essome, who goes by the name Fe Fe. He opened Passage de la Fleur, a natural wine shop in New York, about three years ago. And here's how he defines the difference:

"Organic wine is wine made without any chemicals in the vineyard. Then the issue is that after vinification [the conversion of fruit into fermented wine], the makers have no limitation." For instance, organic winemakers can add sugar and clarifiers (aka fining agents) made with fish and animal products, including egg whites, Essome says.

In his view, natural wine must be unadulterated. But Essome says there isn't much of a way to verify what's behind an organic or natural wine label. While a rigorous certification process exists for labeling organic food, that's "not something that's happening yet in the wine business," he says, adding, "People are trying to fight for that."

That's why he calls the natural wine business something of a Wild West.

"Natural wine is about 1 percent of the wine produced in the world," Essome says, "so it's a fairly big amount. It involves thousands of people, but it's still a work in progress."