KDE attracts many people with different backgrounds. As such, the day-by-day growing KDE community includes many developers, translators, artists as well as usability and accessibility experts and - of course - lots of users.
One main driving force behind KDE is the belief in the open source philosophy. As is defined by the Open Source Initiative Open source is a development method for software that harnesses the power of distributed peer review and transparency of process. The promise of open source is better quality, higher reliability, more flexibility, lower cost, and an end to predatory vendor lock-in. KDE is a defining example of Open source, because of it's high quality, well-known flexibility, lack of price, and collaboration with other Open source projects through initiatives such as freedesktop.org.
Free software is the other main driving force. Despite many similarities, "Open Source Software" and "Free Software" are not the same initiative. As the Free Software Foundation says: Free software is software that gives you the user the freedom to share, study and modify it. We call this free software because the user is free. To use free software is to make a political and ethical choice asserting the right to learn, and share what we learn with others. Free software is usually also Open source software, as Open source software is also Free software. However, Open source focuses more on the technical advantages, whereas Free software focuses more on the ethical aspects. Both philosophies are major driving forces behind KDE development.
A minor and relatively new driving force, Free Culture is also a driving force behind parts of KDE. As it is defined in Wikipedia The free culture movement is a social movement that promotes the freedom to distribute and modify creative works, using the Internet as well as other media. The movement objects to overly restrictive copyright laws, or completely reject the concepts of copyright and intellectual property, which many members of the movement also argue hinder creativity. They call this system "permission culture". The free culture movement holds many of the same ideals as Free Software. All Free Culture supporters support Free Software, though not all Free Software supporters support Free Culture.
Even though some KDE development is done paid, the majority of KDE development is done for free, voluntarily, and on spare time. As a result, the KDE framework is designed to be high-quality, complete, and consistent, and simple KDE programs can be written in very little code.
Despite the fact that most KDE development is done on spare time, some corporations (such as Nokia and Intel) have sponsored funds and/or development time. This sponsorship allow KDE to continue being developed, are used to promote KDE, and provide funds to host events such as Akademy.
KDE is build on top of Qt, a cross platform application framework created by Nokia. While KDE is open source software with libraries licensed under the Gnu Lesser General Public License and applications under the Gnu General Public License, Qt is dual-licensed under a Nokia proprietary license, which costs, and the open source Gnu Lesser General Public License.
Since Nokia provides Qt under the Gnu Lesser General Public License, which does not cost and allows free/proprietary software to link to it, KDE itself gets a high quality application framework for free.
As part of the conditions outlined under the Gnu Lesser General Public License, Nokia provides the source code for Qt under that license as open source. Because of this, many improvements and bug fixes are contributed to Nokia from KDE. Also, since Qt 4.4, the Phonon multimedia layer that originated from KDE is a part of Qt, even though it continues to be developed in KDE. As a result, while KDE gets a free, high quality application framework, Nokia get free, high quality developers and testers.