Post by MarkKordusic

The GPL (which the Linux kernel uses, GPLv2):

• You can freely use, modify, and even sell the software.
• If you distribute your modified version, you must also share the source code under the same GPL terms. This is called "copyleft."
• If you only modify it for private or internal use and never distribute it, you don't have to share anything.
• It's also not exactly "give it to the community." You have to give the source to whoever receives your version, and they can pass it on. You don't have to send it back upstream, though that's considered good etiquette, and it's much easier than maintaining your own fork forever.