Portobello Mushroom (Agaricus bisporus)

Portobello mushrooms are easy to identify because of their large size, which is at least four inches in diameter, and their characteristic brownish color. Portobello mushrooms also have a deep musty smell that makes it seem like they were just picked up from the forest. Portobello mushrooms are white and rounded when young. As they mature, the cap of the mushroom becomes flat and acquires their distinguishing dark color.

Portobello mushrooms are mostly eaten broiled and grilled, but they can also be fried, baked, or sautéed. Because of their size and format, they are often used as a replacement for hamburgers in vegetarian recipes. Portobello mushrooms have a characteristic meaty texture that gets better the longer the mushrooms are cooked, so grilling may be the best way to bring out the flavor.

Portobello mushrooms are specially sensitive to air oxidation, so they should be stored wrapped in a paper towel or in their original container. Refrigerating Portobello mushrooms is ok, but keep in mind that the longer you keep them, the more flavor they lose.

http://www.wisegeek.com/what-are-portobello-mushrooms.htm
http://en.wikipedia.org/wiki/File:Portobello_mushrooms.jpg