Beneficial mycorrhizae (root fungi) live in a symbiotic relationship with 90% of all plants root systems.  Mycorrhizae fungi colonize a plants root zone and the surrounding soil.  These fungi extend microscopic straw-like filaments called "hyphae" into the soil surrounding a host plants root system.  The hyphae then extract, transport, and dramatically increases a host plant's supply of nutrients and moisture.  Nutrients and water once unreachable by ordinary root systems now become accessible through the "mining" effects which mycorrhizae process nutrients for plants.  Mycorrhizae fungi produce acids that process phosphorus and many trace minerals into forms available to plants.  Mycorrhizae fungi can increase the surface absorbing area of a plants root system by 10 to 1000 times facilitating more efficient usage of water and nutrients.  Any nutrients not taken up by plants will remain locked in the Mycorrhizae fungi for later release.  Mycorrhizae fungi are also known to form a protective barrier in and around the root zone outcompeting other organisms for food and space.  

Over time, Mycorrhizae fungi can improve soil quality in the rhizosphere resulting in healthier more disease and drought resistant abundant plants.

Ectomycorrhizae; preferred by hardwood and conifers, form a web colonizing the outside of the root and.

Endomycorrhizae; preferred by most annuals, perennials, vegetables, grasses, shrubs, and softwood trees, colonize the inside of the root and forms strands throughout the soil.

A complete list of well known and often grown plants that form and benefit from a mycorrhizal relationship can be found here... Mycorrhizal Relationship List