Terminology

GNU - GNU (pronounced /gnuː/ (help·info)) is a computer operating system composed entirely of free software. Its name is a recursive acronym for GNU's Not Unix; it was chosen because its design is Unix-like, but differs from Unix by being free software and by not containing any Unix code. (Cited from Wikipedia)

Free Software : “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”.
Free software is a matter of the users' freedom to run, copy, distribute, study, change and improve the software. (Cited from www.gnu.org)

copyleft: Copyleft is a play on the word copyright and describes the practice of using copyright law to remove restrictions on distributing copies and modified versions of a work for others and requiring that the same freedoms be preserved in modified versions. (Cited from Wikipedia)

opensource; pen source is a development method for software that harnesses the power of distributed peer review and transparency of process. The promise of open source is better quality, higher reliability, more flexibility, lower cost, and an end to predatory vendor lock-in. (cited from www.opensource.org)