The licences for most software are designed to prevent users

from sharing or changing it. By contrast, the GNU General

Public License is intended to guarantee the freedom to share

and change free software - to make sure the software is free

for all its users. The GPL is designed to make sure that

anyone can distribute copies of free software (and charge for

this service if they wish); that they receive source code or

can get it if they want; that they can change the software or

use pieces of it in new free programs; and that they know they

can do these things. The GPL forbids anyone to deny others

these rights or to ask them to surrender the rights. These

restrictions translate to certain responsibilities for those

who distribute copies of the software or modify it.