Version 2 of the General Public License, the most well-known of the so-called "copyleft" open source software licenses, achieves its aims by requiring that derivative works based on GPL-based code, such as modifications to that code, be subject to the same terms as the original code; namely, the entire work must be made available in source code form and without royalty or license fee obligations.
The challenge with dealing with the GPL is that the license terms are vague and subject to debate, especially with respect to the question of derivative works. The protector and keeper of the GPLv2, the Free Software Foundation (FSF), takes the position that merely linking with or into GPL code and distributing the combined whole renders the entire work subject to the requirements of GPL – even to the point of claiming that applications running on Linux can be subject to the GPL’s inheritance obligations:
It has always been the FSF's position that dynamically linking applications to libraries creates a single work derived from both the library code and the application code. The GPL requires that all derivative works be licensed under the GPL, an effect which can be described as "hereditary." So, if an application links to a library licensed under the GPL, the application too must be licensed under the GPL. …
While there is some support in the text of the GPL for the FSF’s position, colorable arguments can be made that such activity, without more, is not enough to render the entire combined work subject to GPL, especially if the original GPL-licensed library has not been modified.