Is wordpress free to use?
Yes.
What if I rewrite some fuction and sell it to other people?
Yes, but you are bound by the WordPress license. https://wordpress.org/about/license/
and use of trademarks
http://wordpressfoundation.org/trademark-policy/
WordPress itself is a free product and it branches off into WordPress.com and self-hosted WordPress services.
By using WordPress.com or self-hosted WordPress, you are bound by the following terms of service. https://en.wordpress.com/tos/
To quote from WordPress’s TOS page, “(We’ve made the below Terms of Service available under a Creative Commons Sharealike license, which means you’re more than welcome to repurpose it for your own use. Just make sure to replace references to us with ones to you, and if you don’t mind we’d appreciate a link to WordPress.com somewhere on your website. We spent a lot of money and time on the below, and other people shouldn’t need to do the same.)”
Also, please note that individual themes and plugins made by WordPress contributors will be governed by their own terms. You should be aware of them.
Yes Of course wordpress is free!
But you can’t rewrite some fuction and sell it to other people.
Go through these links for more information:
https://en.wordpress.com/tos/
https://wordpress.org/about/license/
