GNU Public License
The licence, under which the WordPress software is released, is the GPL v2 (or later) from the Free Software Foundation. A copy of the licence is included with every copy of WordPress, but you can also read the text of the licence here.
Part of this licence outlines requirements for derivative works, such as plugins or themes. Derivatives of WordPress code inherit the GPL licence. Drupal, which has the same GPL licence as WordPress, has an excellent page on licensing as it applies to themes and modules (their word for plugins).
There is some legal grey area regarding what is considered a derivative work, but we feel strongly that plugins and themes are derivative work, and, thus inherit the GPL licence. If you disagree, you might want to consider a non-GPL platform such as Serendipity (BSD licence) instead.
Copyright (c) 2013-present Jędrzej Chałubek
Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the “Software”), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions: The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
THE SOFTWARE IS PROVIDED “AS IS”, WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.