A mycorrhiza (Gk.,: fungus roots, pl mycorrhizae, mycorrhizas) is a symbiotic association between a fungus and the roots of a vascular plant. In a mycorrhizal association, the fungus colonizes the host plants’ roots, either intracellularly as in arbuscular mycorrhizal fungi, or extracellularly as in ectomycorrhizal fungi. They are an important component of soil life and soil chemistry.
This mutualistic association provides the fungus with relatively constant and direct access to carbohydrates, such as glucose and sucrose supplied by the plant.The carbohydrates are translocated from their source (usually leaves) to root tissue and on to fungal partners. In return, the plant gains mineral absorption capabilities.
Mycorrhizal plants are often more resistant to diseases, such as those caused by microbial soil-borne pathogens and are also more resistant to the effects of drought.
More info - http://en.wikipedia.org/wiki/Mycorrhyzal