The most popular of the commercially grown edible mushrooms, button mushrooms (Agaricus bisporus) are available fresh in supermarkets year round. Buy them fresh, not canned.
Button mushrooms are creamy white to beige in colour and have a mild flavour; they are usually sold when the smooth round caps are still joined by a veil to the stem. A larger, coffee-coloured variety is the cremini mushroom; when these become very large and the cap is completely separated from the stem, exposing the gills, they become the tasty meaty portobello mushroom.
Button mushrooms grow in dung and so should definitely be rinsed before use. A common kitchen myth is that they should be lightly brushed with a damp paper towel or special mushroom brush; the thinking is that the mushroom easily absorbs water and so will become water-logged if drenched. This is not true. Mushrooms are mostly water, and button mushrooms do not have exposed gills which can store water, so it is not necessary to take great care in rinsing them before use. Just run them under a gentle stream of water, or plunge them into water, taking care to remove any clumps of brown guck that are clinging to the mushroom before using.
Mushrooms are best stored in a paper bag, where they will slowly dry out if not used. In plastic, they will turn to slime.