Lichen is the variously coloured mossy type stuff you often see on trees and rocks. It's composed of a fungus and alga in a symbiotic relationship. There are a LOT of different types.
Lichen is also the name of a skin disease, but let's not go there.
Lichenology is the study of lichens and things which have been lichened.
Personally I pronounce the word litch-in (as in litch-gate which I feel is appropriate because churchyards are a good place to see lichen). The alternative is to say it as though it were written liken.
Lichen forms when something just stands about doing nothing. You can't afford to do that if you want to enter this free crimewriting competition, as it ends at the end of the month. The prize is a two book contract with Avon (an imprint of Harper Collins).