Oakmoss, Evernia prunastri, is a lichen and one of the most commonly used ingredients in both women's and men's fragrances. Lichen is a category of fungi that lives in a symbiotic relationship with hard porous surfaces like trees trunks, branches, etc. and rocks. Sometimes lichens can also co-exist with certain forms of algae.
Tree Mosses, Evernia furfuracea and Usnea barbata are also used quite often in perfumery, but more so in men's fragrances. These are found growing on pine trees or spruce trees and are sometimes misleadingly called oakmoss, when in reality they should be called tree moss or fir moss. The reason they're mislabeled or misrepresented is because sometimes they are substituted as cheaper, inferior alternatives to true oakmoss, which grows only on oak trees, particularly the Quercus robur species of oak. Tree mosses tend not to have the complex scent profile of oakmoss but basically work the same way as a fixative to slow down the rate of evaporation of lighter more transient notes. They tend to provide a dry lichenous, mostly one dimensional background note for fragrances.