The license under which the WordPress software is released is the GPLv2 (or later) from the Free Software Foundation. A copy of the license is included with every copy of WordPress, but you can also read the text of the license here.
Part of this license outlines requirements for derivative works, such as plugins or themes. Derivatives of WordPress code inherit the GPL license. Drupal, which has the same GPL license as WordPress, has an excellent page on licensing as it applies to themes and modules (their word for plugins).
Kuna eneo la kijivu la kisheria kuhusu kile kinachozingatiwa kuwa ni kazi ya derivative, lakini tunahisi vikali kwamba vijalizi na mionekano ni kazi derivative na kwa hivyo kurithi leseni ya GPL. Ikiwa haukubaliani, unaweza kutaka kufikiria jukwaa lisilokuwa la GPL kama vile Serendipity (leseni ya BSD) badala yake.