It's a very diverse group of substances that are common in the plant world. Their common feature that brings together in one group is the presence in the molecule of the aromatic (benzene) core, which is one hydroxyl or phenolic, hydroxyl groups. The aromatic nucleus can have two and three hydroxyl groups, which are distinguished by mono-, di-or tri phenol. These are the main reactive hydroxy groups of phenolic compounds. If the molecule contains several aromatic nuclei with the appropriate hydroxy, polyphenols occur. The simplest is the actual phenol, or carbolic acid, a powerful disinfectant. Catechol, resorcinol and hydroquinone refer to bisphenols, to triphenol - pyrogallol, phloroglucinol and oksigidrohinon. The enzymes are able to polymerize phenols, insoluble molecule of tannins, lignin, melanins and humus.phenols description

When to the simple monomeric phenol is attached a carbon atom with the acid group, hydroxybenzoic acid and its derivatives are formed. When attached three-carbonic combination, a group oxycinnamic acids and coumarin is formed. Compounds, consisting of two aromatic rings and three-carbonic radical are attributed to flavonoids and divided them into groups of catechins leucanthocyanins, flavones and flavonols (yellow dye of colors), and anthocyanins (blue, red and purple pigments of flowers and leaves). In plants, polyphenols are of more complex structure.
Phenolic compounds are formed in all organs of the plant of sugars and involved in processes of respiration in cells, carrying hydrogen molecules from oxidation. In plants, they play the role of metabolic waste, i.e. excreta, which in the course of evolution have proven useful, because of deterring animals from eating the crop, that is, they provide immunity. They reserve substances. Phenolic compounds have a strong effect on plant growth by inhibiting the germination of seeds, elongation of stems and roots. They have strong phytoncidic properties and provide immunity of plants to fungal, and especially to bacterial infection. Often there are no protective phenols in healthy plants, they are formed in it as a reaction to infection with the causative agent of the disease.
Phenolic compounds play an important role in wound healing, cell division, as well as the protection of tissue from ionizing radiation, free radicals, oxidizers and strong mutagens. The content of phenolic compounds in plants varies greatly. Substances such as hydroxybenzoic acid and coumarins are in many plants and some phenols - only in certain plant species. Their content also depends on external conditions. Thus, without ultraviolet irradiation amount of chlorogenic acid abruptly decreases. Growing up in the mountains, where the amount of ultraviolet radiation is higher, the plants contain significantly more anthocyanins and flavonols. This should be taken into account in the procurement of such medicinal plants, where the main role is played compounds, containing phenol.
Flavonoids - derivative of phenolic compounds, yellow, brown pigments of plants. They exhibit different phytotherapeutic action. Found in plants in the form of glycosides and in pure form. Flavonoids that are known in herbal medicine flavonoids: rutin, hesperidin, hyperoside, quercetin, kaempferol and apigenin. flavonoid action
The most significant pharmaceutical effect of the flavonoids is to regulate the flavonoids capillary state, in particular they increase their permeability for atherosclerosis and thereby contribute to reduce and normalize blood pressure. They are credited with diuretic action, choleretic and antispasmodic effects on the human body, they expand capillaries, reduce blood pressure, tone the heart muscle, dilate the coronary vessels, and reduce blood clotting. Flavonoids are found as one of the active compounds in many plants, expanding capillaries, they facilitate the influence of other active compounds. The most significant sources of flavonoids include arnica, birch leaves and butterbur, heather, black elder flowers, chamomile flowers, the color of lime, parsley root, the fruit of the horse chestnut, horsetail and licorice root.
Tannins. Chemically - it is polymerizable phenolic compounds. Their effect is that they are sealed and fixed protein molecules in the surface layers of the skin or mucous membranes, which are therefore resistant against external impacts and less permeable. It's their property is used in the industrial tanning hides. Tannins are readily soluble in water. With protein molecules, such as gelatin, as well as salts and alkaloids, heavy metals, they form insoluble precipitates.
The tannins are divided into two series - pyrogallol and pyrocatechin, depending on which of these compounds lies in their base. According to common classification tannins are divided into condensed and hydrolyzed. Hydrolyzed have in their large polymer molecules ester bonds, which are readily cleaved with the addition of water (hydrolyzed) by enzymes and weak acids, and even boiling. It basically derivatives of gallic or benzoaric acid. Condensed tannins are not broken up, because the relationship between the molecules has different nature. These include tea catechins. Under the influence of strong acids or by oxidation, they form a red phlobaphene products (tea infusion and many plants darkens on standing). The taste of astringent tannins is astringent, therefore the mucous membrane of the mouth, as well as any other mucous membrane, which comes in direct contact of tannins becomes tight, compacted, and because of this they are opposed by the action of inflammatory infections and irritants. Blood flow is reduced, the small capillaries may overlap at all, a painful separation of copious mucus is reduced, too strong motility of the gastrointestinal tract, caused by disorders of the digestive system, slows down. Tannins are applied topically on the skin as bathrooms and lotions for treating facial or scalp, especially for fat and excessive formation of dandruff, in order to stop small bleeding of the skin, against slight burns, for example against sunburn treatment of wounds and frostbite, especially the hands and feet, antiperspirants of feet. Internally they are used for rinsing the mouth with gingivitis and oral cavity, throat, angina, against inflammation of the gastric mucosa and intestine, as well as lavage against diarrhea. However, in diseases of the gastrointestinal tract is first necessary to see a doctor to determine possible infections! Tannins are included in some hemorrhoidal funds (candles, ointments). They are used also as an antidote for poisoning by salts of heavy metals and alkaloids, as these poisons are still in the stomach.
As catechins and flavonoids, tannins, play an important role in   quenching the organism the so-called free radicals, i.e. molecular fragments are formed by the action of radiation and for some other reasons, and having a very high reaction force, destroy important structures of living cells, e.g. DNA. With this, in particular, are related the aging process. “Quenching” free radicals, tannins let us to maintain youth and health.
Especially a lot of tannins in the plants of tropical rain forests, but our flora is also rich in them. The greatest amount of tannins contain cecidium (spherical formation on the leaves of oak, pistachio, arising after the insects lay their eggs in the cecidium). Many of tannins is in sumac, smoke tree, leather bergenia, in the bark of oak and spruce, willow and alder wood and leaves, chestnuts and of herbaceous plants - a mountaineer tanning, Limonium, sorrel and rhubarb.
An important source of tannins in the home is regular black tea, which contains a lot of so-called catechins, which are active after the oxidation of the organism. With a long standing solution of tea, as well as other tanning substances, they are polymerized (condensed) in a reddish-brown insoluble phlobaphene and their activity is decreased. Many of tannins are in the leaf of heather, bilberry and blueberry, peppermint mints, rosemary, sage (in the last three have essential oils!), in the leaves and young fruits of walnut.
Phenolic compounds are responsible for disinfecting and healing properties of propolis - bee glue. Bees collect raw materials for propolis with sticky buds of poplar and other plants.
Anthracene compounds, derived from tricyclic condensed phenols, exhibit a laxative effect on the large intestine. These substances include emodin: phrangulaemodin from rhineberry, reumemodin from rhubarb, aloin from aloe and sennidin from Indian senna. In plants they are in the form of glycosides, which belong to laxative teas that have an effect in 6-10 hours. Sometimes these drugs are taken for weight loss, but it is dangerous because of physiological addiction. The muscles of the intestine become even more flabby, mucosa is damaged  through constant irritation, and therefore it cannot be done.