The OpenZFS software is licensed under the Common Development and Distribution License (CDDL) unless otherwise noted.
The OpenZFS documentation content is licensed under a Creative Commons Attribution-ShareAlike license (CC BY-SA 3.0) unless otherwise noted.
OpenZFS is an associated project of SPI (Software in the Public Interest). SPI is a 501(c)(3) nonprofit organization which handles the donations, finances, and legal holdings of the project.
The Linux Kernel is licensed under the GNU General Public License Version 2 (GPLv2). While both (OpenZFS and Linux Kernel) are free open source licenses they are restrictive licenses. The combination of them causes problems because it prevents using pieces of code exclusively available under one license with pieces of code exclusively available under the other in the same binary. In the case of the Linux Kernel, this prevents us from distributing OpenZFS as part of the Linux Kernel binary. However, there is nothing in either license that prevents distributing it in the form of a binary module or in the form of source code.
Additional reading and opinions: