What is Lichen? Why is it so cool??

On a recent bike trip to the shore of Lake Waubesa near Madison, WI I saw this awesome tree covered in this crazy greenish stuff.

Summer has returned to the northern hemisphere and I'm SO PSYCHED.

That greenish stuff on the trunk of the tree are lichens. Lichens are the coolest fungi! Lichens are also the coolest plants and the coolest bacteria! Wait...  how is that possible????