Lower alcohol wines are generally accepted by the 'wine industry' as wine having an alcohol level of 10.5% ABV or below - though confusingly, some include the 5.5% ABV products, which of course isn’t officially wine.
OIV - The organization's definition of wine states: Wine is the beverage resulting exclusively from the partial or complete alcoholic fermentation of fresh grapes, whether crushed or not, or of grape must. Its actual alcohol content shall not be less than 8.5% ABV (Alcohol by Volume).

NB: (Nevertheless, taking into account a number of specific and unique conditions, the minimum total alcohol content may be able to be reduced to 7% in certain regions.)

 

The difference between one end and the other is rather broad: 10.5% is where many cool climate Rieslings and Prosecco’s rest, all of which are legally wine; the other end is a product that has been de-alcoholised to fit into a markets much lower excise tax duty bracket.
There are number of ways of achieving lower-alcohol wine: some grape varieties are naturally lower in sugar - therefore alcohol - grown in cool climates. There are also technical means which can remove alcohol from the fermented grape juice. Then there is a technique known as early harvest, where grapes are picked slightly early, resulting in less sugar, and therefore less alcohol. The technique where alcohol has been removed - the winemaker has to compensate for the loss of sweetness - some make the mistake and use sugar, which can adversely unbalance the wine.
Grapes have the distinction of having the highest sugar content of any fruit which makes it perfect for fermentation, the process whereby yeast converts sugar into alcohol. Depending on ripeness and sugar levels, most grapes will produce wine that ferments between 10 - 14% ABV. Alcohol is important as it acts as a preservative, is key to the aromatics and personality of a wine, helps with the flavours and mouth-feel, intensity as well as complementing and releasing flavours in cuisine and then to refresh the palate to do it all again.
To create a wine with less alcohol than normal requires special techniques such as fermenting less ripe grapes, retaining sugar in the finished wine, adding back fresh grape juice to dilute the alcohol or using reverse osmosis to remove alcohol through technology. With all of these options - the trick is to produce a lower alcohol wine and make a drinkable wine. For example when you use grapes that were not fully ripe - you risk making an unbalanced wine - or by taking some of the alcohol out of wine made with fully ripened grapes, which can make a wine lacking in character.
Often cool climate wines have a racy acidity and this can be tempered by leaving some sugar unfermented. This hides the acidity and the unfermented juice helps to reduce the alcohol. A wine such as Moscato D’Asti 5.5% ABV is a good example, as they have a potential alcohol if fermented dry of 10%. Which raises an important point: lower alcohol wine isn't necessary low-kilojoule. It can have as many, if not more kilojoules, than regular wine, so be sure to check the label if that is your priority.
Another way of reducing alcohol in wine is to add water to dilute the alcohol level - this practice is frowned upon in Europe, but in America in some regions it is not uncommon. The other more technical methods of separating alcohol from wine; Spinning Cone - Reverse Osmosis - Vacuum Distillation - Low Pressure Evaporation under inert gas.
Whatever method used, wines with their alcohol levels reduced or removed leaves the wine in a less stable state; as such these wines need to be treated with extra care in hygiene standards to ensure the wine is in the best stable condition possible - plus storage after bottling.
For red wines there is a natural limit to how much the alcohol can be reduced. Red wines can’t go down to 5.5% ABV, as the resulting wine will be out of balance - a light-style red wine requires a minimum of 8.5% alcohol to have balance and character.
Other wine classifications include: Alcohol Free: 0 - 0.05%, De-Alcoholised: 0.05 - 0.5%, Low Alcohol Wine: 0.5 - 1.2%, Reduced Alcohol: 1.2 - 5.5%.