Copyleft is a licensing style where much of that which is or can be protected under copyright is given to the users, hence the term in Finnish is fi."käyttäjänoikeus" == en."user's right".
It was first popular in free software but has since expanded to include various other types of works of art or utility, most known of which is of course the Wikipedia.
Most known GNU copyleft licenses are GPL for code and GFDL for other written works
Another famous family of copyleft licenses is the Creative Commons