What is GPL

Section 2(b) of the GPL, in light of the definition of "work based on the Program," is sometimes described as "viral" or "infectious" because it affects any software, even the licensee's own software, that "contains or is derived from" the original GPL-licensed program. A licensee's own software, if it is a "work based on the Program," becomes subject to the terms of the GPL, thereby requiring the licensee to publish his or her own source code.
A "derivative work" is defined in the Copyright Act, 17 USC 101, as a:
"work based upon one or more preexisting works, such as a translation, musical arrangement, dramatization, fictionalization, motion picture version, sound recording, art reproduction, abridgment, condensation, or any other form in which a work may be recast, transformed, or adapted. A work consisting of editorial revisions, annotations, elaborations, or other modifications which, as a whole, represent an original work of authorship, is a 'derivative work'."
Simply combining a copyrighted work with another work does not create a derivative work. The original copyrighted work must be modified in some way. The resulting derivative work must itself "represent an original work of authorship." So if the licensee does not modify the original GPL-licensed program, but merely runs it, he is not creating a derivative work.
The GPL is an attempt to enforce an economic bargain between licensors and licensees. Licensors under the GPL open their source code and distribute their software freely to all who agree to do the same for their own derivative works. If a licensee creates a derivative work by modifying the original GPL-licensed program, or embeds the GPL-licensed program within his or her own program, the resulting work must also be licensed under the GPL. If there are no modifications to the GPL-licensed program, and it is not embedded within a proprietary program, the terms of the GPL simply don't apply to the licensee's software.

Lawrence Rosen is attorney for Open Source Initiative, which manages and promotes the Open Source Definition