After permissions, the most important decision data publishers need to make is around licensing. A clear licence determines what can be legally done (and not done) with the data you make available on your site.
What is a licence?
In this context, a licence is a legal statement of what can and cannot be done with copyright works.
When you provide a licence for others to reuse your data, you are giving them permission to use that data in prescribed ways. You should think carefully about how you license your data, as some forms of licence cannot be revoked.
What is copyright?
Copyright is a property right that applies to certain categories of original works. In most countries, copyright lasts for the life of the author, plus seventy years. While most countries have exceptions for specific kinds of uses, these exceptions are generally restrictive. For most uses of copyright works, permission has to be granted by the copyright owner.
The most prevalent open licence is known as Creative Commons. Creative Commons licences are free, internationally recognised, standardised, and have been adopted by governments, universities, businesses and educators all over the world. There are over a billion works licensed under Creative Commons.
The Creative Commons licences allow you to choose between six licences, ranging from more to less permissive. The less permissive licences restrict commercial and derivative use; the more permissive licence allow commercial reuse, and requires only attribution.
Creative Commons also allows for a Public Domain dedication, called CC Zero. This dedication can be used to give away all related copyrights, to the extent possible under the law.