The four essential freedoms

A program is free software if the program's users have the four essential
freedoms: [1]

    The freedom to run the program as you wish, for any purpose (freedom 0).
    The freedom to study how the program works, and change it so it does your
computing as you wish (freedom 1). Access to the source code is a precondition
for this.
    The freedom to redistribute copies so you can help others (freedom 2).
    The freedom to distribute copies of your modified versions to others
(freedom 3). By doing this you can give the whole community a chance to
benefit from your changes. Access to the source code is a precondition for
this.