A mushroom is the fruiting body of a fungus. It is not a plant, it is a fungus. It eats organic matter and doesn't photosynthesize like most plants do.

They typically grow on soil or on organic matter like woodchips, seeds or compost.
A mushroom is, just like an apple, the fruiting body of a living organism. An apple contains seeds which are needed to reproduce and so do mushrooms. Mushrooms contain thousands of microscopic spores responsible for the reproduction of the fungus.

We can put these various species in one of 4 categories: saprotrophic, mycorrhizal, parasitic, and endophytic.

Saprotrophic mushrooms are decomposers. They release acids and enzymes that break down dead tissue into smaller molecules they can absorb. Thus decaying wood, plants, and even animals can become food for a saprotroph. You can easily see how important saprotrophs are to the food chain! It's no wonder this category includes so many gourmet and medicinal types of mushrooms. Some examples are below:
Morels (Morchella angusticeps, Morchella esculenta, etc) - These elusive, delicious species are very popular with mushroom hunters (see picture at right). Known to be mycorrhizal as well.
Reishi (Ganoderma lucidum) - Highly prized in Chinese medicine, this mushroom is now the subject of many medical studies.
Shiitake (Lentinula edodes) - Famous for both its great taste and medicinal properties.
Portobello/Button (Agaricus bisporus) - Common in supermarkets all over the world. Did you know all 3 belong to the same species?
Cremini (Agaricus bisporus) - Another Agaricus bisporus strain that's a great edible. Come learn the real difference between a cremini and a portobello.
Oyster (Pleurotus ostreatus) - Another popular edible, also known for its cholesterol-reducing effects.
Maitake (Grifola frondosa) - Edible, known anti-tumor properties, and it looks like a brain!
Turkey Tail (Trametes versicolor) - Although too tough to be edible in any manner other than a tea, this is one of the most well-studied medicinal mushrooms.
Giant Puffball (Calvatia gigantea) - These large mushrooms are only edible when young.
Chicken of the Woods (Laetiporus sulphureus) - Younger specimens are known to taste similar to, you guessed it, chicken. Also known to be parasitic.
Enokitake (Flammulina velutipes) - Easy to cultivate and often used in soups.
Shaggy Mane (Coprinus comatus) - This unique looking mushroom has antibiotic properties.
Black Trumpet (Craterellus cornucopioides) - The best tasting edible mushroom out there!
Yellow Houseplant Mushroom (Leucocoprinus birnbaumii) - Famous for popping up in potted houseplants.
Mycorrhizal mushrooms have a fascinating relationship with trees and other plants. The mycelia of these fungi enter into a beneficial union with the roots of plants by either weaving into the root cells (endomycorrhizal) or wrapping around the roots themselves (ectomycorrhizal).

How is this beneficial? The mycelia bring in additional moisture, phosphorous, and other nutrients to their hosts. In return they gain access to sugars (such as glucose) that the hosts produce. This allows plants to grow bigger, faster, and stronger than their nonmycorrhizal counterparts. Many farmers and gardeners will inoculate their crops with a mycorrhizal fungus for better growth.

An estimated 95% of plants form mycorrhizal partnerships with fungi. The types of mushrooms these fungi produce are difficult to cultivate and are often found only in nature. The ones below make a delicious treat if you can find them:

arw Porcini (Boletus edulis) - Often used in soups and sauces, this mushroom can grow quite large.
arw Truffles (Tuber melanosporum, Tuber magnatum, etc) - These gourmet delights are very expensive.
arw Chanterelles (Cantharellus cibarius, Cantharellus formosus, etc) - Another prized edible found on many continents (see picture above).
arw Matsutake (Tricholoma matsutake) - Highly sought after for their flavor and aroma in cooking.
Parasitic types of mushrooms also take plant hosts. Although in this case the relationship is one-sided. These fungi will infect the host and eventually kill it.
Sometimes the line between parasitic and saprotrophic is not so clear. The honey mushroom is a known parasite yet it will also continue to live saprotrophically on the dead wood of its host. Most true parasitic fungi do not produce mushrooms and are too small to be noticed on a tree until it's too late. Some notable types of mushroom producing parasites are:
arw Honey Fungus (Armillaria mellea, Armillaria ostoyae, etc) - Some species in the Armillaria genus (see pic at right) are edible, some are bioluminescent, and one colony is suspected to be the largest organism on the planet!
arw Caterpillar Fungus (Cordyceps sinensis) - A true parasite that preys on insects. This interesting mushroom may just be my favorite.
arw Lion's Mane (Hericium erinaceus) - This strange specimen possesses spiny teeth instead of the traditional cap. In addition to being edible, it's also suspected to help heal nerve tissue!

Endophytic fungi deserve their own category due to their behavior. Endophytes partner with plants by invading the host tissue. However, unlike with parasitic fungi, the host remains healthy and seem to benefit with increased nutrient absorption and resistance to pathogens. Unlike mycorrhizal fungi, most endophytes can be easily cultivated in a lab without their host present.

Benefits / uses
Mushrooms have been shown to reduce the risk of hormone-dependent breast cancer, according to a study from a research center in California. The fungi contains a compound shown to inhibit estrogen which can fuel the growth of breast cancer and ovarian cancer.

Additionally, mushrooms have niacin, and just getting 22 mg of niacin each day can reduce the risk of Alzheimer's by 70%. Different types of mushrooms also provide different benefits.

Following is a list of 4 types of mushrooms with specific benefits:
1 - Button Mushrooms - Helps you lose weight - Those popular little white mushrooms sold in practically all supermarkets have belly-reducing/flattening properties. Button mushrooms have a type of carbohydrate that helps steady blood sugar levels while keeping metabolism high. Consuming 3 ounces of button mushrooms each day will help you burn more calories and shed as much as 13 pounds in five weeks, according to a study.

2- Maitakes - Helps keep cells free of cancer - Maitakes are the mushrooms shown to reduce breast cancer. By eating 1/2 cup daily, you can reduce the risk of any cancer by as much as 40% - and protect your body from hormone-triggered cancers like breast cancer and ovarian cancer. Maitakes have compounds that causes abnormal cells to self-destruct while activating the natural killer cells in the immune system.

3 - Shiitakes - Protects you from infections - A natural compound in shiitakes, known as lentinan, revs up white blood cells to help fight off infections. Eating 5 ounces per day will triple the ability of your body to fight off bacteria and viruses, according to researchers.

4- Reishis - Protect your heart - The active ingredient, gandodermic acid, helps control cholesterol levels by 12%, can help shrink the plaque causing clogged arteries by 2/3, and can reduce blood pressure by 12%.Eating just a few ounces per day will relax the arteries and keep them clear.

Research studies / References
arw Click Here for: Mushrooms and Health - On Line Article
arw Chang R, 1996. Functional properties of edible mushrooms. Nutr Rev 54(11), S91-S93
arw Chilton, J. 1993. What are the health benefits of mushrooms? Let's Live, Dec., pp. 24-29.
arw Dharmananda, S., 1988. Medicinal Mushrooms. Bestways Magazine, July, pp. 54-58.
arw Hobbs, C. 1997. Overcoming Chronic Fatigue (Traditional remedies for a modern disease). Veggie Life, Jan. Vol 5: #5, pp. 56-59.
arw Jones, 1997. An ancient Chinese secret promotes longevity and endurance. Healthy & Natural Journal, vol. 3, issue 3, pp. 90-93.
arw Law, David, 1996. Fungi as a platform for new medicine. Mushroom World, December, 1996.
arw McDougall, H. 1998. Detoxification. Veggie Life, March, pp 30 - 35.
arw Mohammed, Gina, 2010. Boost Your Immunity with Gourmet Mushrooms - Shiitake, Maitake and Reishi pack potent disease-fighting compounds. The Herb Companion August/September 2010
arw Smith, C. 1994. Gold medal herbs. Natural Health May/June, pp. 85-87
arw Smith, J,., Rowan, N., Sullivan, R. 2001. Medicinal mushrooms: their therapeutic properties and current medical usage with special emphasis on cancer treatments. Shirota, M, 1996. What You Should Know About Medicinal Mushrooms. Explore! vol. 7, issue 2, pp. 48-52.
arw Stanislaus, C., 1996. Ling zhi- medicine of kings. New Editions Health World, pp. 38-41.
arw Steinman, D., 1995. Potent protectors. Natural Health, Nov-Dec. pp. 92-95; 134-135.
arw Weil, A. 1993. Boost immunity with mushrooms. Natural Health, May-June, pp. 12-16.
arw Wiley, C. 1991. The medicinal side of mushrooms. Vegetarian Times, March 1991.
arw International Journal of Medicinal Mushrooms Editor-in-Chief Solomon P. Wasser