The most common definition of symbiosis is a mutually beneficial relationship between two different species closely interacting with each other. These interactions tend to be long term with the survival of both species hinging on the interactions. The most common example of this form of mutually beneficial symbiotic relationship in plants apart from the lichen is the mycorrhiza.

The mycorrhiza is a symbiotic relationship established between fungi and the roots of plants, although a mycorrhizal relationship can occur in plants that do not have roots. Examples include psilotum and bryophytes (mosses and liverworts).

The lichen symbiosis is an association between fungi and algae which develop into distinct morphological forms where the alga develops into a phycobiont and the lichen becomes the mycobiont.

Symbiosis may also be classified according to the type and level of interaction between species. When one of the organisms benefits at the expense of the other over time the relationship is described as parasitic. When an organism benefits but not at the expense of the other, the relationship is described as commensalism. A third type, the phoresy, is a loose association between the species where a smaller organism uses the larger one as a transport host.

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