The Four Freedoms
WordPress comes with some awesome, worldview-changing rights courtesy of its license, the GPL.
The 1st Freedom
To run the program for any purpose.
The 2nd Freedom
To study how the program works and change it to make it do what you wish.
The 3rd Freedom
To redistribute.
The 4th Freedom
To distribute copies of your modified versions to others.
WordPress grows when people like you tell their friends about it, and the thousands of businesses and services that are built on and around WordPress share that fact with their users. The WordPress community is flattered every time someone spreads the good word, just make sure to check out the WordPress Foundation trademark guidelines first.
Every plugin and theme in WordPress.org’s directory is 100% GPL or a similarly free and compatible license, so you can feel safe finding plugins and themes there. If you get a plugin or theme from another source, make sure to ask them if it’s GPL first. If they do not respect the WordPress license, it is not recommended to use them.