This is a broad introduction to many species but a valuable introduction to make. Saying, "Meet Lichen" is like saying, "Meet Birds". There are many many different kinds of lichen but the differences between species is simply astounding and more than you probably want to know. But simply knowing about Lichen is a good place to start this introduction. First of all, lichen is not a plant and not a fungus and not moss, although it looks a little like all three. Lichen has no roots, but instead produces it's own nutrients through photosynthesis like a plant. Lichens often live ON plants but they are not parasites, they take nothing from the plant. Lichens live in a symbiotic relationship with fungi and bacteria but are themselves, neither.
Lichens can live in some of the most extreme locations on earth from the arctic tundra to deserts. You may find lichens on rocks, pavement, treebark, the ground, sand, pretty much anywhere. We have them growing on a metal table on the front porch. Lichens are ancient too, one was aged at 8,600 years old making it one of the oldest living organisms on earth. They have the ability to essentially dry out and freeze completely and then when it is damp out, the fill back out and continue to photosynthesize.
Lichens come in all shapes, sizes and colors but in the northern US, we often see greenish grey lichen growing on trees. Look closely though and you may find orange lichen on rocks, green lichen on downed trees, grey lichen on barn siding. Lichen grows slowly. Very very slowly. Like 1-2 millimeter a YEAR. It is also an indicator of air quality. Areas with high levels of air pollution have fewer lichen species. Lots of lichen = better air quality.
It can look like moss sometimes, with it's own branchy looking growth or it can grow with moss. It can be spotty in appearance, or scaly and flat. It really is a unique and interesting organism we don't tend to notice but take a moment to look closely at a log or branch or a rock near you and I bet you'll find some.