Dextrose is a simple sugar made from corn that is 100% fermentable. Dry malt extract, on the other hand, is about 75% fermentable. Dextrose often used for priming bottle conditioned beer. Typically 140 – 200 g are added to prime a 23L (5 gallon) batch. Dextrose has a place in making the most economical beer kits possible (1 kg / canned kit), although we really prefer high quality malt sources such as Coopers Brew Enhancer 1 or 2, Dry Malt Extract, or Muntons Kreamy X for a much fuller flavour. Using to much sugar (1 kg / 23 L) like many older kits recommended will impart a “cidery” taste into your homebrew and is not recommended.
Many high gravity beers add a dash of dextrose as a fermentable source, usually around 10% or less of the fermentable sugars. Dextrose will lighten the body and color of the beer as well.