Fresh basil is used primarily as a fresh herb in cooking. When served with tomatoes, it develops its characteristic flavor. However, it is also used as a medicinal plant. Basil not only promotes digestion, but also calms our nerves and helps relieve migraines.
From Wikipedia: “Basil (Ocimum basilicum), also called great basil or Saint-Joseph's-wort, is a culinary herb of the family Lamiaceae (mints). It is also called the "king of herbs" and the "royal herb". The name "basil" comes from Greek βασιλικόν φυτόν (basilikón phutón), "royal/kingly plant".”
“The various basils have such different scents because the herb has a number of different essential oils that come together in different proportions for various breeds. The strong clove scent of sweet basil is derived from eugenol, the same chemical as actual cloves. The citrus scent of lemon basil and lime basil reflects their higher portion of citral, which causes this effect in several plants including lemon mint, and of limonene, which gives actual lemon peel its scent. African blue basil has a strong camphor smell because it contains camphor and camphene in higher proportions. Licorice basil contains anethole, the same chemical that makes anise smell like licorice, and in fact is sometimes called "anise basil." ”
See the link above for other chemicals that help to produce the scents of many other types of basils.
“Basil is most commonly used fresh in cooked recipes. In general, it is added at the last moment, as cooking quickly destroys the flavor. The fresh herb can be kept for a short time in plastic bags in the refrigerator, or for a longer period in the freezer, after being blanched quickly in boiling water. The dried herb also loses most of its flavor, and what little flavor remains tastes very different, with a weak coumarin flavor, like hay.
Basil is one of the main ingredients in pesto—a green Italian oil-and-herb sauce. ... The leaves are not the only part of basil used in culinary applications, the flower buds have a more subtle flavor and they are edible.”
“As a medicinal plant, it is known as Basilici herba (lat.: of the basil herb or plant). Basil is also used in folk medicine, in particular, in the Mediterranean region, to treat loss of appetite (stomachicum) and bloating and gas (carminative), and is sometimes also used as a diuretic, lactagogue, and for gargling in cases of pharyngitis.
The essential oil has anthelmintic (deworming) and antiphlogistic (anti-inflammatory) properties and inhibits the development of stomach ulcers.*”
“Basil is possibly native to India, and has been cultivated there for more than 5,000 years. It was thoroughly familiar to the Greek authors Theophrastus and Dioscorides. It is a hardy annual plant, best known as a culinary herb prominently featured in Italian cuisine, and also plays a major role in Southeast Asian cuisines of Indonesia, Thailand, Malaysia, Vietnam, Cambodia, Laos, and Taiwan.”
“Basil has religious significance in the Greek Orthodox Church, where it is used to sprinkle holy water. The Bulgarian Orthodox Church, Serbian Orthodox Church, Macedonian Orthodox Church and Romanian Orthodox Church use basil (Bulgarian and Macedonian: босилек; Romanian: busuioc, Serbian: босиљак) to prepare holy water and pots of basil are often placed below church altars.
In Europe, basil is placed in the hands of the dead to ensure a safe journey. In India, they place it in the mouth of the dying to ensure they reach God. The ancient Egyptians and ancient Greeks believed it would open the gates of heaven for a person passing on.”
Note (italics): * = Translation from a German Wikipedia entry