A lichen is a symbiosis. That means that it is two or more organisms living together such that both are more successful within the partnership than they would have been if they were living on their own. With lichens the basic components of this partnership are 1) a fungus called the 'mycobiont' and 2) one or more algae and/or a cyanobacteria called the 'photobiont'.
The true nature of the symbiosis between this two partners is still being debated by scientists and some would maintain that the fungus is a parasite on the photobiont. However, in many cases, the algae in question cannot survive alone in the habitat occupied by the lichen any more than the unattached fungi can, so it is not realistic to use the term parasite.
The fungal partners are mostly (over 95%) Ascomycetes. Most of the rest are Basidiomycetes. As far as science has been able to discover few if any of the fungi involved can survive and reproduce in the wild on their own. Each lichen species contains a different species of fungi and so it is according to the species of fungi that lichens are classified. This classification is generally based on characteristics of the thallus and reproductive organs. There are between 13500 and 17000 species of lichen depending on whose classification you believe. About 20% of fungal species are involved in lichen partnerships.
The algal partners are far less numerous than fungal partners. In other words any given species of algae will probably form part of several if not many different lichens. Many, if not all the algal partners can exist on their own in some habitats, however, normally when part of a lichen they have a much greater distribution.
The fungal partner benefits by getting sugars, its only nutriment from the algae which being green synthesises sugars through photosynthesis. The algal partner gets protection as the fungi normally forms the outer surface. This protection is against the weather mostly, it results in the algae having a more stable and constant environment to live in allowing it to grow better. The fungi collects the sugars by means of special hyphae called appressoria or haustoria which contact the wall of the algal cells. The fungi may produce a substance which increases the permeability of the algal cell walls such that they cells lose as much as 80% of the sugars that they produce. The sugars pass into the algal hyphae through diffusion.
Morphologically, lichens are made up of a few distinct characters. The most obvious is the thallus. The form of the thallus is a result of the fungal species involved. The thallus is the body of the lichen. Most of what you see, if it isn't reproductive structures, is thallus. The fungal hyphae (filaments) branch and then fuse together (anastomose) when they meet to form a mesh of hair-like threads. The top surface is normally a layer of tightly packed hyphae called a 'cortex'. Below this is the algal layer where the photobiont lives. Below this is the medulla an area of loose hyphae in which nutrients are stored. Sometimes a lower cortex exists, in others the medulla rests on the surface. In crustose and squamulose lichens there is no lower cortex. In foliose lichens there is a lower cortex and in fruticose lichens the lower cortex is replaced by a central one.
Filamentous lichens are totally different. They consist of chains of algal cells wrapped around with fungal hyphae. Because nature was not designed to fit into our classification schemes some things inevitably to do not fit. In lichens it is the genus Cladonia. Cladonia are a successful group of lichens which have a primary thallus and a secondary thallus. The primary thallus is small and clings closely to the substrate while the secondary thallus is a shrubby growth like fruticose lichens. Once the lichen is established the primary thallus often dies off.
Crustose lichens, as their name implies, form a crust on the surface of the substrate on which they are growing. This crust can be quite thick and granular or actually embedded within the substrate. In this latter case the fruiting bodies still rise above the surface. In many crustose lichens the surface of the thallus breaks up into a cellular, crazy-paving like pattern. Crustose lichens tend to grow out from their edges and have their fruiting bodies in their centre. Crustose lichens are very difficult to remove from their substrates.
Some lichens have a portion of their thallus lifted off the substrate to form 'squamules'. They are otherwise similar to crustose lichens in that they possess an upper cortex but no lower cortex.
These have an upper and lower cortex. They are generally raised to some extent above the substrate but connected to it by rhizines (specialised root-like hyphae). They are easier to remove from their substrate when collecting because of this.
Fruticose lichens are shrubby lichens. They are attached to their substrate by a single point and rise, or more usually, dangle from this. Some foliose lichens can be stubby like fruticose lichens, however, close examination will reveal that the algal part exists only on one side of the flattish thallus whereas in fruticose lichens it exists as a ring around the thallus, even when it is flattened as in Ramalina sp.
Leprose lichens are an odd group of lichens which have never been observed to produce fruiting bodies. Because knowledge of the form of the fruiting bodies is essential to the identification of fungi, these lichens have not yet been identified properly, or at least not yet given full scientific names. These fungi not only lack an inner cortex, but also lack an outer one, i.e. no cortex, only an algal cell layer and sometimes a weakly defined medulla.