From Brewing Quality Beers by Byron Burch:
Sugars are added to the recipe in order that the yeast may ferment and turn the sugar into alcohol. Sugars that are not maltose (sugar from the malt) is considered adjuncts.
Typically in a recipe, there will be at least 70% of maltose and no more than 30% of the sugars from adjuncts. The more sugar you substitute for malt, the lighter in both color and taste the beer will be.
Kinds of Adjuncts:
Directly supply sugar-
Dextrose (Corn Sugar)-
most commonly used adjunct, carries less potential for creating off-flavors (like cane sugar); in large amounts, it will give beer a "cider-y" taste.
Sucrose (Cane Sugar)-
produces slightly higher Gravity (more alcohol) than corn sugar, but has a "hotter" taste, so it is less useful when brewing delicately flavored beers; in large amounts, it will give beer a "cider-y" taste.
Lactose (Milk Sugar)-
non-fermentable; used to sweeten some stouts, but has a flavor that doesn't agree with lighter flavored beers (use dextrin powder instead to add smoothness and a hint of sweetness). Add by cooking a sugar syrup.
Molasses (Brown Sugar)-
used in small amounts for flavoring dark ales.
gives beer a crisper, drier flavor than corn sugar; added to boiling pot; may take longer to clarify in bottle; substitute for corn sugar 1:1
Indirectly supply sugar (Unmalted Grains):
Flaked grains: flaking prepares the starch in the grain for conversion to fermentable sugars when exposed to enzyme in barley malt. Should always be mashed with equal or greater amount of lager or Pale malt
no more than 30% of fermentable sugar should be from flaked rice; make sure it is fresh; used in place of rice syrup
for oatmeal stout; no more than a pound per five gallon batch
helps with head retention; gives a grainy character; no more than 8 oz per 5 gallon batch
Flaked Maize (Corn) or Yellow Corn Grits:
used like flaked rice, but with a slightly different effect.
If using cereal, watch out for preservatives (they're not nice to yeast).