An adjunct is nothing more than a non-malt source of fermentable sugars. Thus, this broad term includes. Belgian candi syrups of all colors. Unmalted wheat, barley, rye, oats, maize, and other grains. Honey, maple syrup, agave nectar, and other sugars.
The reasons for adding an adjunct are varied. Some brewers will seek them for head foam retention, flavour or even to cut costs! To adjust for this, brewers who are looking to brew a high ABV beer, will add a yeast nutrient to give the yeast some respite from all the sugar.
American Adjunct lager is the best selling style of beer in the nation, and encompasses all of the giant brands: Budweiser, Miller, Coors, Pabst, etc. There are three general categories that American-style lagers fall into: Light lagers, standard lagers, and premium lagers.