The simple version: wine with nothing added to it and nothing removed from it. 

Here's the spark notes for the natural winemaking process:

1. Crush organic and/or biodynamic grapes
2. Ferment the grape juice spontaneously without adding any commercial yeast
3. Mature the wine in inert vessels: used old oak barrels, concrete, or stainless steel, or fiberglass tanks
4. Do not filter the wine (this is why natural wines tend to be cloudy and often time has sediment in the bottle)
5. If any at all, add MINIMAL EFFECTIVE sulfites.
6. Bottle the wine. 

In order to create alcohol, you need sugar and yeast inside a vessel. The yeast metabolizes the sugar resulting in a byproduct, alcohol.

Conventional vs. Natural Winemaking.

Conventional Winemaking Process:

• Step 1: Grow grapes with chemical "fertilizers." 
• Step 2: Harvest the grapes with machines without human sight, taste, or touch to differentiate between high and low quality grapes
• Step 3: Mechanically crush the grapes aka make grape juice
• Step 4: Add sugar to the grape juice if your grapes do not have enough naturally occurring sugar for fermentation to take place
• Step 5: Ferment the grape juice using lab bred COMMERCIAL/CULTURED YEASTS 
• Step 6: INSERT ADDITIVES. There are ~100 ingredients that the FDA allows producers to put in their wine. However the FDA does not require them them to be listed on the label. The common culprits are irresponsible levels of sulfites, acidifiers, synthetic tannins, wood chips (instead of using pricier oak barrels), coloring agents (such as mega purple), and the list goes on and on! 
• Step 7: Fine and filter the wine
• Step 8: Age/mature the wine (or pretend to with shortcuts like oak chips). Often conventional winemakers use new oak barrels because they impart more "wood" flavors and mask the purity of the grapes themselves.

Natural Winemaking Process:

• Step 1: Grow grapes using organic, biodynamic, and regenerative agriculture practices.
• Step 2: Humans harvest the grapes by hand. Machines are not natural.
• Step 3: Crush grapes aka make grape juice (this is often done via the old-fashioned "foot stomping" method as opposed to a mechanical crush).
• Step 4:  Ferment the grape juice using NATIVE/WILD YEASTS
• Step 5: Add small amounts of sulfites or none at all. Learn more about sulfites here.
• Step 6: Age/mature the wines in neutral vessels such as stainless steel tanks, concrete eggs/tanks, or used old oak barrels (after 1 use, the wood flavors die off considerably and so on and so forth)
• Step 7: Drink that shitttt!!!!

Conventional winemakers use shortcuts that their natural counterparts do not. Natural winemakers need to measure twice and cut once, while conventional winemakers have more wiggle room for "corrections". So let's back up for a second and discuss the key differences. 

1. Natural growers practice organics and other techniques that require a lot of grit and hard work. Conventional growers use chemical fertilizers because it's easier.
2. Natural wine is hand harvested. Conventional wine is mechanically harvested.
3. There is never added sugar in natural wine. Some conventional wineries chaptalize (add sugar) to the grape juice/must.
4. Natural winemakers deploy native fermentations using indigenous yeasts that naturally dwell in the vineyard and the winery. Conventional winemakers use lab-grown yeast strands.
5. Natural winemakers may or may not add a SMALL amount of sulfites. Conventional winemakers irresponsibly add way too many sulfites and other artificial additives. **Remember sulfites are a natural byproduct of fermentation. All wines have sulfites in them.**
6. Natural winemakers leave their wine unfined and unfiltered (remember the "nothing added / nothing removed" philosophy?). Conventional winemakers typically fine and filter before bottling.
7. Natural winemakers rarely use new oak barrels and NEVER use wood chips. Conventional winemakers commonly use both.