Generally, Open Source software is software that can be freely accessed, used, changed, and shared (in modified or unmodified form) by anyone. Open source software is typically made by many people, and distributed under licenses that comply with the definition.
Free Software is an older and related term that basically says the same thing for all our intents and purposes, but we stick to the term Open source in this document for simplicity.
curl and libcurl are distributed under an open source license known as a MIT license derivative. It is very short, simple and easy to grasp. It follows here in its completeness:
COPYRIGHT AND PERMISSION NOTICE
Copyright (c) 1996 - 2015, Daniel Stenberg, <daniel@haxx.se>.
All rights reserved.
Permission to use, copy, modify, and distribute this software for any purpose
with or without fee is hereby granted, provided that the above copyright
notice and this permission notice appear in all copies.
THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT OF THIRD PARTY RIGHTS. IN
NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM,
DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE
OR OTHER DEALINGS IN THE SOFTWARE.
Except as contained in this notice, the name of a copyright holder shall not
be used in advertising or otherwise to promote the sale, use or other dealings
in this Software without prior written authorization of the copyright holder.
This is basically legalese that says you are allowed to change the code, redistribute the code, redistribute binaries built from the code and build proprietary code with it, without anyone requiring you to give any changes back to the project - but you may not claim that you wrote it.
Copyright is a legal right created by the law of a country that grants the creator of an original work exclusive rights for its use and distribution.
The copyright owner(s) can agree to allowing others to use its work by licensing it. That's what we do in the curl project. The copyright is the foundation which the licensing works on.
Daniel Stenberg is the owner of most copyrights in the curl project.
The curl project obeys to national laws in which it works. It being a highly international project visible, downloadble and usable in effectively every single country on earth may of course imply that some local laws can be circumvented when using curl. That's just the nature of it and if uncertain, you should check your own local situation.
There have been law suits involving technology that curl provides. One such case known to the author of this was a patent case in the US that insisted they had the rights to resumed file transfers.
As a generic software component that is usable everywhere to everyone, there are times when libcurl - in particular - are used in nefarious or downright malicious ways or circumstances. Examples include being used in virus and malware software. That is unfortunate but nothing we can prevent.
As contributors and maintainers of this project, we pledge to respect all people who contribute through reporting issues, posting feature requests, updating documentation, submitting pull requests or patches, and other activities.
We are committed to making participation in this project a harassment-free experience for everyone, regardless of level of experience, gender, gender identity and expression, sexual orientation, disability, personal appearance, body size, race, ethnicity, age, or religion.
Examples of unacceptable behavior by participants include the use of sexual language or imagery, derogatory comments or personal attacks, trolling, public or private harassment, insults, or other unprofessional conduct.
Project maintainers have the right and responsibility to remove, edit, or reject comments, commits, code, wiki edits, issues, and other contributions that are not aligned to this Code of Conduct. Project maintainers who do not follow the Code of Conduct may be removed from the project team.
This code of conduct applies both within project spaces and in public spaces when an individual is representing the project or its community.
Instances of abusive, harassing, or otherwise unacceptable behavior may be reported by opening an issue or contacting one or more of the project maintainers.
We encourage everyone to participate in the development of curl and libcurl. We appreciate all the help we can get and while the significant portion of this project is source code, there are a lot more than just coding and debugging help that is needed and useful.
We develop and discuss everything in the open, preferably on the mailing lists.
The source code to curl and libcurl have also been provided and published publicly and it continues to be uploaded to the main web site for every release.
Since March 2010, the curl source code repository has been hosted on github.com. By being up to date with the changes there, you can follow our day to day development very closely.