Pronounced li-ken, lichens aren't exactly plants and they're not exactly fungi. Lichens are actually a complex symbiotic partnership of several different organisms. A symbiotic partnership is one in which two, or more, different organisms interact in a mutually beneficial way.
Lichen, therefore, is a symbiosis between fungi and algae, or fungi and cyanobacteria, but most of the time between all three.
The fungus is the dominant partner in the relationship and is ultimately responsible for creating the structure of the lichen and determining what it looks like.
So why do these organisms need to live in partnership?
Fungi are heterotrophic which means they don't contain chlorophyll and can't photosynthesize or feed themselves in any way. Algae and cyanobacteria, conversely, can both photosynthesize - using energy from the sun to manufacture glucose (this makes them autotrophic) - but are unable to survive on their own outside of water. The perfect pair!
Fungi provide the stable condition on which algae and cyanobacteria can grow on land (a home), and the algae/cyanobacteria, in turn, provide simple sugars (food) to the fungus. Win, win.
So, why isn't lichen a plant?
A fungus is not a plant because its cell walls aren't comprised of cellulose and it doesn't contain chlorophyll with which to photosynthesize. A fungus, therefore, is scientifically classified to be in the Kingdom Fungi.
Algae is also technically not a plant (say what?!), despite their ability to photosynthesize. This is basically because algae are unable to survive on their own outside of water. Algae are thus placed in the Kingdom Protista.
And cyanobacteria? Well, put on your safety goggles and get ready to time travel back to sixth grade science class, because cyanobacteria are actually prokaryotic. This means that cyanobacteria, although often called blue-green algae, are not eukaryotes the way that plants, fungi, or (in fact) algae are. Eukaryotic organisms have cells that contain a membrane-bound nucleus, whereas bacteria - and cyanobacteria - don't. Yep, cyanobacteria have no nucleus or membrane-bound organelles (making them prokaryotic), but they're still able to photosynthesize because they contain chlorophyll. Cyanobacteria are, consequently, the only bacteria to contain chlorophyll. But, despite their (super handy) photosynthesizing abilities, cyanobacteria are classified in the Kingdom Monera - with the rest of the bacteria.
And plants are, well, plants. They're all classified in the Kingdom Plantae.
So, if you think about it, lichen is pretty cool in that its symbiotic nature unites three different kingdoms of classification!
Why does any of this matter?
Because lichen is the only true vegan - and palatable - source of Vitamin D.