The simple answer to the question, “What is Drupal?” is: Drupal is a web publishing system. Drupal is a program, or a web application, used to manage content on a website.
But this is only one part of the real answer. Drupal is not only a tool for managing content on a website, but a tool to build tools to manage content on websites. It is a meta tool used by web developers to build customized web publishing tools. In some respects, you may even call Drupal a programming language.
But even this is not the full answer. Most people who have used Drupal for a while will tell you that Drupal is a community: a group of people spread out over the world, sharing a common goal or interest, and feeling that they have something connecting them. This community has more than half a million members, in one way or another contributing to the project called Drupal. Some put their efforts into making Drupal look better (an appreciated contribution). Quite a few people code and build new functionality. Some work with documentation, translations, or answering questions from people new to Drupal. Some people in the community are so well known that they almost could be compared to rock stars. But the vast majority of the community consists of people who mostly just use Drupal as a publishing tool. They help the project every once in a while by posing new questions, reporting a bug or bringing feature requests to developers.
Several of the people who will be most important to Drupal's growth and development in the future have just started using it.
Drupal is distributed with a license usually called open source. It means that in contrast to nearly all other publishing systems of the same magnitude, Drupal is free of charge. There is no fee to download or start using Drupal, and no yearly license payment to continue to use it.
More fundamental than the price, though, is what you are allowed and not allowed to do with Drupal. If you read the fine print of end-user licence agreements for software, you will see that they require you to agree to not install the program on more than a certain number of computers, not hack or reverse-engineer the software, and not violate any of the patents the software includes.
Open-source licenses were created as a reaction to the limits that closed-source software creates. Open-source licenses explicitly say that you may use the program as much as you like, you may fiddle with its code to find out how it is built, you may share copies of the software to your friends, and you may use the software (or parts of it) to make new and better software. If you use Drupal to build a website, you can be sure that you can write plugins to help your site work with other systems you use, without being sued.
The philosophy behind open source is really quite simple: It is better if we work together.
For quite a long time, open-source software was pretty insignificant in comparison to other paid software systems – it was mostly used by computer nerds with weird beards. Today, one of the top ten websites in the world (Wikipedia) is run on open-source software, and is filled with information that is also governed by open-source licenses. Also, one of the top web publishing systems in the world is shared as open source. We call it Drupal.