Most people and even some growers don't know what mycorrhizal fungi are. It is a fungus that forms a mutualistic and helpful relationship with plant roots. Mycorrhizal fungal filaments in the soil function as extensions of plant root systems and are more effective in nutrient and water absorption and delivery than the roots themselves. In essence, they bond with and feed the roots, which in turn feed your plants. 90% of plants in the wild benefit from Mycorrhizae and your plants are missing out if you are not supplementing your soil with these amazing fungi.
Mycorrhizae are usually classified as either ectomycorrhizas and endomycorrhizas, and live in symbiosis with the plants they come in contact with.
In Ectomycorrhizal Fungi, the hyphae (the root like structures that spread out in soil) do not penetrate individual cells within the plant roots, while the hyphae of endomycorrhizal fungi penetrate the cell wall of the roots and inhabit the cell membrane.
In simple terms, ectomycorrhizal fungi serve to uptake and move nutrients around the area they populate, providing multiple plants in the area with nutrients throughout the fungal network.
Endomycorrhizal fungi go a step further in that they actually "invade" the cell walls of the plants living within their fungal network, increasing the surface area of contact, thus making possible a more efficient method of delivering an increased amount of nutrients right into the plant roots.
While some plants benefit more from a specific type of mycorrhizae (there are many, many types), the best bet for your plants could be a mix or balance between two or more types of mycorrhizae.
If you are growing with a sterile or store bought soil, soil-less mix, or even hydroponically, chances are you are missing out on mycorrihizae. You will be doing your plants and yourself a big favor by supplementing some in your garden!