This is simply a difference on how fizz is introduced to beer. During fermentation yeast cells naturally create carbon dioxide gas in beer. During bottling or kegging process more carbon dioxide (CO2) can be added. Most beers are carbonated using CO2. But some are carbonated using nitrogen (N2). Nitrogenated beers are not purely N2. It’s a mixture. Usually 70% N2 and about 30% CO2.
Nitrogenated beers have smaller bubbles and thus have more stable foam. A very good example of this is Guinness.