Disaccharides are formed when 2 monosaccharides are linked together by a glycosidic linkage.
Maltose used for beer brewing is glucose + glucose.
Lactose, found in milk, is glucose + galactose.
Sucrose, table sugar, is actually glucose + fructose.
Plants (including fruits and vegetables) usually carry carbohydrates from their leaves to other parts in the form of sucrose.
When we eat sucrose, it is quickly broken down into glucose and fructose, causing a rapid rise in blood glucose levels (not good for diabetics!) Polysaccharides (starch) is broken down much more slowly, so it helps us maintain steady blood glucose levels (and keeps us feeling fuller for longer!).