[Overview]
The term "copyleft" was invented in 1984 by Richard Stallman, who was a
computer scientist at MIT (Massachusetts Institute of Technology).
"Copyleft is a general method for making a program or other work free,
and requiring all modified and extended versions of the program to be
free as well. (...) Copyleft says that anyone who redistributes the
software, with or without changes, must pass along the freedom to
further copy and change it. Copyleft guarantees that every user has
freedom. (...) Copyleft is a way of using of the copyright on the
program. It doesn't mean abandoning the copyright; in fact, doing so
would make copyleft impossible. The word 'left' in 'copyleft' is not a
reference to the verb 'to leave' — only to the direction which is the
inverse of 'right'. (...) The GNU Free Documentation License (FDL) is a
form of copyleft intended for use on a manual, textbook or other
document to assure everyone the effective freedom to copy and
redistribute it, with or without modifications, either commercially or
non commercially." (excerpt from the GNU website)