Linus Torvalds explains that the FSF snuck some stipulations into the GPL v3 which breaks the spirit of the GPL v2:
There is a Wikipedia has an article on tivoization, and here is a tutorial guide on copyleft and the GPL v2 and v3. There’s also a question on Stackoverflow that has some useful info, although it was closed as off-topic. From answerer Will M:
The page linked to in another answer is a good source, but a lot to read. Here is a short list of some the major differences:
- internationalization: they used new terminology, rather than using language tied to US legal concepts
- patents: they specifically address patents (including the Microsoft/Novell issue noted in another answer)
- “Tivo-ization”: they address the restrictions (like Tivo’s) in consumer products that take away, though hardware, the ability to modify the software
- DRM: they address digital rights management (which they call digital restrictions management)
- compatibility: they addressed compatibility with some other open source licenses
- termination: they addressed specifically what happens if the license is violated and the cure of violations
I agree with the comment about consulting a lawyer (one who knows about software license issues, though). In doing these things (and more), they more than doubled the length of the GPL. Although GPLv3 is a complex legal document, it was designed to be read and reasonable understood by software developers. There is also a guide to understanding it and in depth discussion of the changes from v2 to v3 at http://copyleft.org/guide/.
From answerer TT23:
GPLv3 of June 29, 2007 contains the basic intent of GPLv2 and is an Open Source license with a strict copyleft (→ What types of licenses are there for Open Source software, and how do they differ?) However, the language of the license text was strongly amended and is much more comprehensive in response to technical and legal changes and international license exchange.
The new license version contains a series of clauses that address questions that were not or were only insufficiently covered in version 2 of the GPL. The most important new regulations are as follows:
a) GPLv3 contains compatibility regulations that make it easier than before to combine GPL code with code that was published under different licenses (→ What is license compatibility?). This concerns in particular code under Apache license v. 2.0.
b) Regulations concerning digital rights management were inserted to keep GPL software from being changed at will because users appealed to the legal regulations to be protected by technical protective measures (such as the DMCA or copyright directive). The effectiveness in practice of the contractual regulations in the GPL has yet to be seen.
c) The GPLv3 contains an explicit patent license, according to which people who license a program under the GPL license both copyrights as well as patents to the extent that this is necessary to use the code licensed by them. A comprehensive patent license is not thereby granted. Furthermore, the new patent clause attempts to protect the user from the consequences of agreements between patent owners and licensees of the GPL that only benefit some of the licensees (corresponding to the Microsoft/Novell deal). The licensees are required to ensure that every user enjoys such advantages (patent license or release from claims), or that no one can profit from them.
d) In contrast to the GPLv2, the GPLv3 clearly states that there is no requirement to disclose the source code in an ASP use of GPL programs as long as a copy of the software is not sent to the client. If the copyleft effect is to be extended to ASP use (→ When does independently developed software have to be licensed under the GPL?), the Affero General Public License, Version 3 (AGPL) must be applied that only differs from the GPLv3 in this regard.
There’s a website which helps as a guide to choosing a license.
Torvalds also offers some thoughts on earning respect:
An interesting point of view, indeed.