A few of the sugars you know and love are actually disaccharides -- pairs of simple sugars linked together into a package deal. Sucrose, lactose and maltose are perhaps the best known of them. Sucrose is especially important because it serves as a form of energy storage in plants; as table sugar, it's almost ubiquitous in the modern diet. Your body has some enzymes it uses to break disaccharides down.
Disaccharides can't enter the cells in your body intact, so they need to be split apart into their component subunits before they can be properly absorbed. The reaction that breaks them up is called hydrolysis, a word that comes from the Greek for water-splitting. As the name implies, a molecule of water splits during the reaction, with one hydrogen ion going to one of the two liberated simple sugars, or monosaccharides, and the remaining oxygen atom and hydrogen atom from the water molecule going to the other monosaccharide.
At room temperature and without a catalyst, disaccharide hydrolysis is so slow it doesn't take place at an appreciable rate. Strong acids act as a catalyst to accelerate this reaction, and the hydrochloric acid in your stomach will indeed hydrolyze some of the disaccharide molecules in your food. Many of them, however, will remain intact. To break down these remaining disaccharide molecules, your body employs molecules called enzymes -- proteins that act as catalysts to speed up specific reactions.
In your small intestine, disaccharides encounter enzymes attached to the surface of the cells in the intestinal lining, or epithelium. The enzymes catalyze hydrolysis of these disaccharides; meanwhile, the epithelial cells expend energy to actively take up the monosaccharides and transport them inside. From here they will pass into the bloodstream, which carries them throughout your body. Hydrolysis of maltose or trehalose liberates two glucose molecules; sucrose contains a fructose and a glucose, while lactose contains glucose and galactose.
Lactose, a sugar found in milk, is perhaps the most problematic of the disaccharides. You need a specific enzyme called lactase to chop the two sugars in lactose apart. Some adults lack the necessary enzyme and thus are unable to digest lactose completely. The remaining lactose ends up in the large intestine, where various bacteria are only too happy to eat it. These bacteria produce copious quantities of gas as a byproduct, potentially causing diarrhea or severe cramps. Adults without the lactase enzyme are said to have lactose intolerance.