Freedom 1: The freedom to study how the program works, and change it so it does your computing as you wish. Access to the source code is a precondition for this.
Freedom 2: The freedom to redistribute copies so you can help others.
Freedom 3: The freedom to distribute copies of your modified versions to others. 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.
-A lot of free software uses the GNU General Public License (GPL). By using it, you support the free software movement and empower people through software.
+Free software uses the GNU General Public License (GPL). Many legacy programs use GPLv2 while most new free software uses the GPLv3. By using it, you support the free software movement and empower people through software.
This is just the tip of the iceberg. To read more about free software, start with the "What is Free Software?" page on GNU's website.