The field mushroom (Agaricus campestris) was once a very common fungus but it is now declining. It is the original edible mushroom but specimens are now much harder to find and so cultivated species now form the basis of supplies in our supermarkets.
The field mushroom occurs on fairly rich soils, usually amongst grass and are often in troops (large groups) in late summer and in to the autumn. They first appear as 'buttons' but soon grow to have the distinctive smooth, white cap with black or very dark brown gills. The cap can be anything from 5 to 10 cms across.
As I said, they are good to eat but you need to be absolutely certain that what you are picking are field mushrooms otherwise you may not live to pick another!