Natural wine ican be thought of simply as wine with little added or taken away. The concept becomes complicated when you look at the varying levels of interference and chemical additions used in all other wines.
Often people will assume that organic or biodynamic wine is low in additives or even additive free but these terms refer mainly to the farming of the fruit. Imagine buying an organic apple pie made with organic apples but was also full of artificial flavours, colouring and preservatives. You probably wouldn't buy it and because food labelling laws insist on full disclosure such products don't exist. For some reason wine doesn't have these laws.
Isabelle Legeron writes that "[m]ost people regard wine as an artisan product that is made from grapes using simple equipment... people don't realise its a lot more complicated than that. In fact, apart from sulfites, eggs, and milk, most additives, processing aids, and equipment are used behind closed doors because of shortcomings in wine-bottle-labeling laws... wine-makers are often reluctant to discuss exactly what is used when, even if what they are doing is completely legal. The result is that the industry as a whole is mostly cloaked in secrecy." (Natural Wine, 2014).
One key labelling law you've probably noticed is that all wines state 'contains sulfites'. This is a fairly redundant disclosure. Sulfites are a bi-product of natural fermentation. Usually there will be anything up to 20mg per litre of naturally occurring sulfites. Sulfites are also used as an additive for sterilisation and stabilisation. The added sulfite is a man-made product (usually a by-product of the petro-chemical industry). A wine with added sulfites may have levels up to 40 times higher than a wine with naturally occurring sulfites, but there will be no legal labelling term that differentiates the two.
Sulfites are the most contentious topic in the natural wine debate. Many would claim good wine can not be made without them. Some would claim that good wine can not be made with.
There is more to natural wine however than sulfites or the lack of them. Natural wines are made with wild, indigenous, natural or ambient yeasts. Essentially, these terms refer to the yeasts that exist on the grape at harvest. Most wines are made with selected yeasts. Selected yeasts are cultured in a laboratory to give the wine-maker more control in flavour and aroma profile. They are usually selected to give a quick and reliable fermentation. Use of selected yeasts is an intervention and can create more elaborate results, but some might argue an integrity and purity are lost in the resulting wine. It is also worth noting sulfites are required to kill the already present wild yeasts.
Another element is filtration. A lot of natural wines are un-fined or un-filtered. This sometimes means the resulting wine is cloudy. This cloudiness is created by fruit pulp and is no different to added flavour and texture you might find in cloudy cider or cloudy lemonade.
It is worth stating that 'natural' is not a legal definition. Its more of a movement. A back-lash against in-adequate labelling laws and undisclosed industrial production. By using little else but grapes and being completely open about wine-making processes, Natural wine-makers have created a lot of waves and ruffled many feathers in the industry. The provenance and the quality of natural wine has created a movement that continues to grow.