Lichen looks like a plant but it is not a plant,it is fusion between algae and fungi.algae is a thallophyte which can prepare its own food by photosynthesis which is called photobiont.fungus can't prepare its own food and depends on others for,fungus harvests sugar from algae by releasing some chemicals into algae for its survival,so,it is called symbiosis,only one organism(fungus) is profited as it feeds on algae.
this is symbiotic relationship between algae and fungus in lichens.
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Symbiotic relationship may be defined as a close association between two living organism of different species, for a long period of time.

Lichens is an excellent example ofsymbiotic association. It consist of a fungus and and its photosynthetic partner,a green algae.
Fungi are heterotrophic in nature so they are unable to produce their own food. In Lichen, fungus provides a tough water proof thallus (body) , to alga. By the help of this partner( fungus) alga can grow on extreme habitat like rocks and tree trunks. Fungus also helps to obtain water and dissolve mineral from the rock which are then used by algae to produce carbohydrate by the process called photosynthesis.