Four principles of Free Software Foundation

Free software is a matter of liberty, not price. To understand the concept, you should think of free as in free speech, not as in free beer.

The four principles are,

• The freedom to run the program, for any purpose (freedom 0).
• The freedom to study how the program works, and adapt it to your needs (freedom 1). Access to the source code is a precondition for this.
• The freedom to redistribute copies so you can help your neighbor (freedom 2).
• The freedom to improve the program, and release your improvements to the public, so that the whole community benefits (freedom 3). Access to the source code is a precondition for this.

http://www.gnu.org/philosophy/free-sw.html
http://www.oreillynet.com/linux/blog/2007/10/the_four_freedoms_applied_to_s.html?CMP=OTC-0O724Z062301&ATT=The+Four+Freedoms+Applied+to+Software+as+a+Service