A license will define what others may or may not do with your data.
We strongly encourage you to assign a license to your work, no matter how or where you share it.
You may choose to assign a broad license that allows anyone to do whatever they like with your data, like a CC0 license, or you may assign a more narrow license, such as one that restricts their use to strictly non-commercial activities and requires attribution of the data creator whenever it is used.
The two primary current sources for licenses for research data are Creative Commons and Open Data Commons. If you elect to deposit data into the Stanford Digital Repository (see below), you will have the option to choose any of these licenses or one of several software licenses. See the full list of licenses available in the SDR. Other repositories may have other options; there are hundreds of possible existing licenses.
A number of licenses are also available specifically for open source software (OSS). If you are interested in an OSS license but aren't sure how to pick one, choosealicense.com may be of help to you.
If your data includes content from other sources, you should be sure that the license you choose works with any licenses on that material. For example, if your data includes content with a license that has a "share-alike" provision, then your dataset will likely also need to have a "share-alike" provision, since that's how that provision works!
You can license your work even if it's not in a repository. You do not need to register the work anywhere, but you do need to make sure that you have the right to apply the license (i.e. no one else has rights to the work) and that you've chosen a license that is appropriate. Then mark your work with the license. Creative Commons provides guidance on how to apply one of their licenses.