Glycosides are a large and varied group of plant metabolites found universally distributed in higher plants and also in some lower plants. They are defined by the common property of yielding sugars among the products of their hydrolysis. Chemically they are considered sugar ethers as they are formed by the condensation of the –OH group of the sugar with the –OH group of a nonsugar component referred to as aglycone. The aglycone or the sugar component is most frequently glucose with rhamnose, fucose, cymarose, and digitoxose being among the other sugars. Based on the α or β configuration of the sugars, both forms of glycosides are possible. However, only β forms do occur in plant glycosides, with α-linkages being common among carbohydrates.

Some glycosides contain more than one sugar being linked to each other as a disaccharide or trisaccharide. In many glycosides, the sugar could be a sugar derivative such as glucuronic acid, galacturonic acid, etc.

When the glycosidic linkage between the sugar and nonsugar takes place through oxygen, they are O-glycosides. Such glycosides are easily hydrolyzed to the parent sugar and the aglycone by either enzymes or acids. When the linkage is between the –OH of the sugar and the –CH group of the aglycone they are C-glycosides. These glycosides resist normal acid hydrolysis and can be successfully hydrolyzed only by oxidative hydrolysis with ferric chloride. N-glycosides are those in which the linkage is formed between the amine group of the aglycone and the OH group of the sugar and S-glycosides are formed through linkage of the –OH group of sugar with –SH group of the aglycone.

Aglycone of a glycoside includes representatives of many numerous groups of hydroxyl compounds occurring in plants ranging from small molecules to tetracyclic and pentacyclic triterpenoid moieties, hydroxyanthraquinones, naphthaquinones, anthocyanins, thiocyanates, cyanogenetic, phenylpropanoid, flavones-related compounds, coumarin, furanocoumarin, etc.

Classification of glycosides may be done under different criteria such as the nature of the aglycone, nature of the sugar, nature of the glycone–aglycone linkage, or on therapeutic property.

Some biologically important groups of glycosides based on the aglycone are as follows:

1. Anthraquinone glycosides: Sennosides from Senna leaves and pods, Cascarosides from Cascara bark, Aloin from Aloe sps, rhein, aloe-emodin from Rhubarb rhizome.
2. Cardiac glycosides: These are based on C23 or C24 steroids called cardenolides and bufadienolides respectively. For example, Digitalis glycosides from digitalis leaves, strophanthus glycosides from strophanthus seeds are cardenolides. Toxic glycosides of red and white squill are bufadienolides.
3. Saponin glycosides: are based on either tetracyclic or pentacylcic triterpenoid moiety. For example, dioscin from Dioscorea tubers, ginsenosides form Ginseng roots, Glycyrrhizin from liquorice stolons, Quillaia saponins from soap bark, senegin from Senega roots, etc.
4. Cyanogenetic glycosides: yield HCN on hydrolysis. For example, Amygdalin from bitter almonds, manihotoxin from poisonous manihot root, linamarin from linseed, Prunasin from wild cherry bark.
5. Thiocyanate glycosides: Based on algycones with thiocyanate (SCN) grouping, for example, sinigrin from black mustard.
6. Flavonol glycosides: based on flavone and its derivatives, these occur both in the free state and as glycosides. They form the largest group of naturally occurring phenols. These coloured compounds are widely distributed in nature. For example, rutin from buck wheat, hesperidin from citrus fruits, silymarin from silybum sps, etc.
7. Coumarin and furanocoumarin glycosides: For example, visnagin from dried ripe fruits of Visnaga, psoralen from dried ripe fruits of Psoralea, etc.
8. Naphthaquinone glycosides: For example, lawsone from Henna leaves.
9. Phenol glycosides: Aglycones are based on phenolic groups. For example, salicin from Populus sps, vanillin from vanilla sps, tannin glycosides, etc.

Glycosides accumulate in plant organs at definite stages of growth and characteristic glycosides are found in maximum proportion in young organs in the cell elongation stage. Physiologically glycosides have an important role to play in the metabolism, self defense, and excretory functions of the plant. Because of the large variety of compounds involved, glycosides vary widely physically, chemically, and pharmacologically. They include many therapeutic compounds with an array of biological activities, such as cardiac glycosides, purgative anthraquinone glycosides, flavonoid glycosides, saponin glycosides, etc.