Herbs are plants used for flavouring or medicine and are usually grouped into a number of families, each known by a technical name of Latin origin. Technical names can be confusing, as some have changed in recent years because the scientists who specialise in plant classifiication [taxonomists] have reformed the terminology used.
1: Umbelliferae: This is a large family of herbs that include some very popular ones. Parsley [Petroselinum] is an important member that has exists in several different species. There are also fennel, coriander, chervil, sweet cicely and chervil. Fennel was one of the nine sacred herbs of the Anglo-Saxons. Fennel leaves, roots and seeds are all edible. An infusion of fennel is commercially available and is used to relieve wind in babies. Coriander has edible leaves and seeds, but they have different flavours.
There are some poisonous umbellifers, so you should be careful about wild picking. While cow parsely, which grows wild in hedgerows, is edible, a related species growing with it is not. Aethusa cynapium is a poisonous umbelifer with an acid taste that burns the mouth. Worse, there are some very dangerous umbellifers. Hemlock and Hemlock water Dropwort, for example, are deadly poisonous. These poisonous umbellifers have purple blotches on the stem. In general I would not pick any wild parsley/umbellifer that grows in wet, boggy places such as marshes and ditches. I never pick any wild umbellifer, as I prefer to grow my own.
An exception is Alexander, a herb introduced by the Romans which has tended to frow within range of the sea, often along road sides, although it has been moving inland recently. The move inland may be due to the fact that it tolerates salt, and roads have been salted in winter recently, leading to conditions favourable to alexander. This is an umbellifer that grows early in the year and the leaves can be picked wild. It has a pink tinge to its leaf base.
2: Labiatae: This family includes the many varieties of mint. It also includes sage, marjoram, oregano, thyme, savoury and rosemary. Most mints are grown in gardens. The problem is that mint is very vigorous and can be invasive unless kept under check. Some growers prefer to keep mint in containers.
One member of the mint family deserves care. Pennyroyal, a relatively rare variety not commonly grown nowadays should not be given to pregnant women, as it can induce miscarriage.
3: Lilliacae. This family used to be known as the Alliums. They belong to the broader family that includes lilies, but beware, lilies are poisonous, so not all of this family is edible. The several kinds of onion, leeks, chives, shallots and garlic are all members of this family.
Garlic stands out as a natural healing substance. During the first world war garlic growers were producing it in vast quantities to supply the hospitals, as it contains a powerful antibiotic, allicin. It also contains antifungal and antiviral substances. It was probably an awareness of its healing powers that made people use it as a charm against vampires.
4: Brassicaea. This family used to be known as the Cruciferae. It includes cabbage, kale, sprouts, cauliflowers, broccoli, turnip and kohl rabi etc. most of the family is used as bulk foods rather than as herbs, but there are two important herbs within the family. Mustard is one, and there are several varieties of it. Horse radish is another. In general this family prefers a slightly more limy soil than is needed for other herbs, so pH of 7-7.5 is useful, whereas other families may need a pH of 6.5.
5: Zingiberaceae: This is the ginger family and is known more as a spice than a herb, though the distinction is artificial. The family includes ginger, cardamom and turmeric. Ginger is known for its ability to soothe the digestion and has the advantage that it has no known side effects. In general the family has a range of herbal uses besides being tasty.
Herbs in general prefer a light, well drained soil. They are easy to grow in gardens and can be cultivated in pots on window ledges. Cultivation is far preferable to foraging where these herbs are concerned, as they are not common in the wild. Occasionally they can be found in abandoned, negelected gardens. For example, alexander is often seen near old monastic sites. If you wish to forage for anything, be careful and purchase a good guidebook.