Copyleft is an application of copyright law but its aim is to preserve the "free" nature of the licensed work.
For now, see Wikipedia's definition of copyleft.
In the software world, copyleft is defined as such by the Free Software Foundation:
"Copyleft is a general method for making a program free software and requiring all modified and extended versions of the program to be free software as well."
"In the GNU project, our aim is to give all users the freedom to redistribute and change GNU software. If middlemen could strip off the freedom, we might have many users, but those users would not have freedom. So instead of putting GNU software in the public domain, we 'copyleft' it. 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."