Button mushrooms are actually just one species of the more than 200 species in the genus Agaricus. They are bisporous mushrooms (Agaricus bisporus syn. Agaricus brunnecens and Agaricus hortensis).
General information about mushrooms:
From “en.wikipedia.org/wiki/Agaricus”: “Agaricus is a genus of mushrooms containing both edible and poisonous species, with possibly over 300 members worldwide. The genus includes the common ("button") mushroom (Agaricus bisporus) and the field mushroom (Agaricus campestris), the dominant cultivated mushrooms of the West.
Members of Agaricus are characterized by having a fleshy cap or pileus, from the underside of which grow a number of radiating plates or gills on which are produced the naked spores. They are distinguished from other members of their family, Agaricaceae, by their chocolate-brown spores. Members of Agaricus also have a stem or stipe, which elevates it above the object on which the mushroom grows, or substrate, and a partial veil, which protects the developing gills and later forms a ring or annulus on the stalk.”
General information (Agarius bisporous):
From Wikipedia: “Agaricus bisporus is an edible asidiomycete mushroom native to grasslands in Europe and North America. It has two color states while immature—white and brown—both of which have various names. When mature, it is known as portobello mushroom, often shortened to just portobello.
When immature and white, this mushroom may be known as common mushroom, button mushroom, white mushroom, cultivated mushroom, table mushroom, and champignon mushroom. When immature and brown, this mushroom may be known variously as Swiss brown mushroom, Roman brown mushroom, Italian brown, Italian mushroom, cremini or crimini mushroom, brown cap mushroom, or chestnut mushroom.”
Nutritional value (button mushrooms):
“In a 100-gram serving, raw white mushrooms provide 93 kilojoules (22 kilocalories) of food energy and are an excellent source of the B vitamins, riboflavin, niacin, and pantothenic acid. Fresh mushrooms are also a good source (10–19% DV) of the dietary mineral, phosphorus.
While fresh A. bisporus only contains 0.2 micrograms (8 IU) of vitamin D as ergocalciferol (vitamin D2), the ergocalciferol content increases substantially after exposure to UV light.”
Other varieties of button mushrooms:
“It is the most common edible mushroom in the world and is cultivated in a number of different sizes and color variations. ... Pavement mushrooms (Agarius bitorquis) can have a very similar appearance. Clustered mushrooms (A. cappellianus) have a brown cap and hanging ring. Coastal mushrooms (A. litoralis) also have a hanging ring. ... There are a number of additional cultivated varieties.*”
“A. bisporus is now cultivated in at least seventy countries throughout the world. Global production in the early 1990s was reported to be more than 1.5 million tons, worth more than US$2 billion.”
“This mushroom is commonly found worldwide in fields and grassy areas following rain, from late spring through to autumn, especially in association with manure. It is widely collected and eaten, even by those who would not normally experiment with mushroom hunting.”
“Button mushrooms are the most important cultivated edible mushrooms in the world. They were first cultivated at the time of Ludwig the XIV in Paris by Olivier de Serres. Thanks to the high levels of vitamins, protein, and fiber, the low fat content, and the resulting low caloric value, mushrooms offer many health benefits.*”
Note (italics): * = Translation from a German Wikipedia entry