First of all in all alcohol-free beers you will still find a little alcohol, so low alcohol might be a better term. Alcohol is an essential ingredient of the brewing process and its absence explains the different taste.
Beer starts as a mixture of water and malt that contains sugars. Yeast is used to turn the mixture into alcohol and CO2 bubbles. This is the basis of beer and hard to avoid.
Beer that contains virtually no or little alcohol can be brewed.
One method uses a different kind of yeast that prevents alcohol being formed during the brewing process. Moreover, the yeasting process is halted before alcohol is formed. Alternatively, beer is brewed in the traditional fashion, but the alcohol is extracted.
Heineken 0.0 for instance is produced by allowing alcohol to evaporate. Brugse Sportzot is made my filtering out the alcohol using a special membrane. A combination of the two procedures is used to produce e.g. Jupiler 0.0.
Brewers use creativity to cancel out the loss of taste. Other ingredients are added to help their brew taste like an ordinary pilsner. These ingredients brewers keep secret.
In a test carried out by the consumers’ organisation Test Aankoop Bouwers sold by Albert Heijn came out tops, while a VRT probe selected Bavaria 0,0% as your tastiest buy.