Disaccharides are formed when two monosaccharides are joined together in a process known as condensation that leads to the loss of a water molecule.
The bond is formed between hydroxl groups (-OH) from each sugar molecule. An hydroxl group (-OH) from one sugar combines with a hydrogen atom from the other to form water. The remaining oxygen forms a bond between the two molecules thus forming the disaccharide ('di' means 'two'). This bond between the two sugars is called a glycosidic bond.