Currently there are over 10,000 known types of mushrooms. That may seem like a large number, but mycologists suspect that this is only a fraction of what's out there! We can put these various species in one of 4 categories: saprotrophic, mycorrhizal, parasitic, and endophytic. These categories describe how the the organism feeds itself.
(Note: If you're more interested in how YOU feed yourself, check out the edible mushrooms page).
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.
Think of all the dead matter on the ground. Now imagine what would become of it if there were fewer organisms to recycle it into compost or soil. 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:
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:
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:
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.
Successful cultivation aside, much is still unknown about this category of fungi. Many species do not produce mushrooms and their partnership with plants is not fully understood. Some mycologists suspect that certain parasitic and saprophytic fungi will reveal themselves as endophytes as the field expands. Time will tell what discoveries will emerge as this group is studied further.
For a more in depth look at these types of mushrooms I suggest visiting this page from the University of Sydney.