You propably have heard the word "Copyright", and you might know that it is the legal rights that someone has for his own creations. The fact that a writter has the right to sell copies of his work for some revenue and noone else can use it without having the permission from the creator, but there is also the opposite, or nearly the opposite word, "Copyleft"!
Copyleft is a licence that allows anyone to distribute creation with only some simple limitation, like that "every copy should be distribute along with the licence and the acknowledge of the creators".
