There is a great description on the About WordPress page. However, if you really need to ask then the rest of this page isn't for you - yet - because it's intended for people who are already familiar with the product and want to do more with it.
Originally targetted at blogs, WordPress is also good for a number of other types of web sites. This is because of the (very) wide variety of presentation styles available through themes and the ability to alter - not just add - functionality through plugins. A complete refresh of a site's presentation (look and feel) can be achieved by simply changing the theme.
WordPress is a great tool even though the out-of-the-box version is actually quite basic - clean and robust, but basic. There are three things that add the extra to this ordinary.
Writing your own plugin can be a lot of fun, satisfying and sometimes downright frustrating - the usual experience when dealing with software development. You can keep the plugin for your own use or share it with the world - lots of simple ones never reach the market because they are literally half a dozen lines. Generally the best ones are straightforward to use and born out of necessity because someone the developer knows needed to do something. This kind have two huge benefits over those where the developer either had the need themselves or imagined a gap in the market:
I've created some tutorials showing how to write a plugin, enjoy.
WordPress make it clear on their license page that they believe any themes or plugins are derivative works. If this is true then such works must be licensed under the same GPL license as WordPress - this is a condition of their license to you. In support of this belief, the same license page cites Drupal's License FAQ. However, whilst this provides a lot of useful information, it doesn't address the issue of derivative works beyond stating that such extensions "... are a derivative work of Drupal." There's nothing provided to support this claim. Whether or not these are legally considered a derivative work will only be determined by testing this in a court of law. Until then, it's moot.
Having said all of the above, what is completely reasonable is the position of both WordPress and Drupal that they won't accept, for publication and sharing via their sites, any extensions not licensed under GPL - this is spelt out in the WordPress Detailed Plugin Guidelines and the Drupal License FAQ.
We're not lawyers so you'll find nothing here that is considered legally informed. What I will say is GPL was created as part of the GNU project, the place where linux came from. From there it is a very small step to realise the original purpose of GPL was to promote and ensure the free sharing of software. To that end one aspect of GPL is whilst the copyright ownership of your original work remains with you, anyone can modify it and/or create derivative works from it. The whole GPL is a lot longer than this paragraph so there is a lot more to it, but that's the essence around which the rest of it is built.