Low-carb beer is simply beer that has lower carbohydrate content. An example of this could be Corona Premier. The carbohydrate content in such drinks is converted into carbon dioxide and alcohol, leaving very little sugar and calories behind.
For those of us looking to loose a few pounds or cut down on carbs, low calorie home brewing gives us a chance to be creative, cut down the calories and still enjoy our favorite beers. An average 12 oz (1/3 liter) serving of average strength beer contains about 150 calories.
Because the majority of calories come from alcohol, you can also brew low calorie versions of beers like English Bitters, Milds, as well as Scottish Light ales. The trick in this case is to not only slightly lower the original gravity but also use a high temperature when mashing.
Of course, the key to driving down carbs is hydrolyzing all the available starches with amyloglucosidaise. That's how the recipe can start with a 1.030 OG and still get an ABV greater than 4% and an FG < 1.000. Watch out Miller Lite and Michelob Ultra!