Small carbohydrate molecules are called sugars. Lifeforms like these, they're easy to absorb and use for energy quickly. Fruits are delicious to many animals because of the sugars they contain, and the sweet taste of sugar encourages their consumption.
Sugars are carbohydrate molecules composed of either 1 or 2 of the building blocks of carbohydrates, these are known as either monosaccharides or disaccharides respectively. The usual sugar involved in biochemical reactions is glucose, this is monosaccharide that contains a 6 carbon molecule (a hexose) with 12 hydrogen and 6 oxygen atoms, C6H12O6, which form a hexagonal structure. This sugar is used directly in cellular respiration, and in a slightly different form (β-glucose) is found in cellulose, the structural molecule in plant cells.
Another monosaccharide is fructose. Fructose is another hexose, with an identical chemical composition to glucose, however fructose adopts a slightly different chemical structure, having a pentagonal ring instead of the hexagonal ring found in glucose. Fructose is sweeter to taste than glucose, which means it's used as a sweetener (though especially in the US, due to various corn subsidies). A third common simple sugar is galactose. Galactose is another hexose with a hexagonal structure, similar to glucose, however some of the positions of the atoms are slightly different in galactose. Galactose is used to form glycoproteins found in nerve tissue. Galactose is about as sweet as glucose, but as a monosaccharide isn't widely found in the diet, though it is found in a disaccharide, called lactose (see below). These common monosaccharides are shown in Figure 1.
Figure 1: Structures of common biological monosaccharides glucose, fructose and galactose. Glucose and galactose form hexagonal ring structures, which fructose forms a pentagonal structure, even though all of the sugars have the same chemical formula C6H12O6. Glucose and galactose are stereoisomers, which differ in the orientation of the hydrogen and -OH groups around the 4 carbon in the glucose ring, which gives it different chemical properties. Not pictured here is another stereoisomer of glucose, β-glucose, which is found in cellulose.
Disaccharides are widely found in the human diet (Figure 2). A disaccharide composed of two glucose molecules is called maltose. The major use of maltose is in brewing. Beer is made from malted barley, where the grains have been allowed to germinate before brewing, which breaks down the long polysaccharides into disaccharides which are easier to ferment. Maltose is also what gives the British bedtime drink Horlicks its distinctive taste.
Figure 2: Structures of common disaccharides maltose, sucrose and lactose. All of these disaccharides are formed when a condensation polymer forms between the 1 carbon of a glucose molecule, and the 4 carbon of another monosaccharide, with maltose being formed with another glucose, sucrose being formed with a fructose, and lactose being formed with a galactose.
A disaccharide with one glucose and one fructose molecule is called sucrose. This is well known as table sugar, the white, crystalline form of which is pure sucrose. This is the sweetest of the disaccharides, but it is not as sweet as fructose, but is used as a benchmark against which the sweetness of other sweet molecules are compared. Sucrose is also what gives sweetness to maple syrup, and many other natural sweeteners.
A disaccharide with both glucose and galactose is called lactose. This is the sugar that is found in milk. This is much less sweet than either sucrose or galactose. Lactose has dietary significance as many people (in fact most people outside of western nations) have lactose intolerance. Lactose requires a specific enzyme, called lactase to digest it. Most people (globally) do not consume milk into adulthood and thus do not keep producing this enzyme, which is produced in childhood to digest milk.
Longer chain carbohydrates are called oligosaccharides and polysaccharides. These are often used to store glucose for energy. They don't taste as sweet, but are found in the diet as a source of energy and are stored as a long term store of energy.