Data Licensing is an important part of sharing your data. Even if the funder requires you to open or share your data, a license is a courtesy to potential reusers so they know up front what they can do with it. It increases the likelihood that other researchers will consider your data for reuse, which is a positive for you. Simply releasing the data without a license creates ambiguity. Different countries have differing rules around data IP (Intellectual Property), and Copyright. Data itself (the raw numbers) can't be copyrighted, but databases, charts, and tables can. For example in the US creativity is important, so maybe that table of sensor data can't be copyrighted. But in the EU the act of compiling the data is deemed sufficient for copyright. Since you have no idea who might be interested in using your data, it makes sense to provide guidance in the form of a license.
There are a few common terms which will help you to understand data licenses. Attribution is the requirement that a reuser must give credit to the creator - you. Often it stipulates that it is required for any distribution, display, performance, or use in a new work. Copyleft requires that any new work derived from your data has to be released under the same license. This can create problems when a researcher compiles data from several sources that have conflicting licenses. Non-commercial requirements limit a user to using the data for non-commercial purposes. That runs into the problem of what constitutes a commercial endeavor. A License is a legal instrument that enables the data owner/creator to provide permissions to other users to use the dataset under specific terms. Remember that to apply a license you MUST own the data. Be sure to check with your institution if you are not sure.
The Digital Curation Centre (DCC) has an excellent section on How to License Research Data, including a summary of the CC and ODC licenses. The ODC's Guide to Open Data Licensing includes sections on the legal framework in the EU, Canada, and US. The Australian National Data Service (ANDS) provides a Licensing and copyright for data reuse guide and FAQ.
Choose a license. There are many licenses available to choose from. There is no one-size-fits-all solution. It depends on the data type, what you will or will not allow others to do with it, what the funder or institution requires, what your discipline uses, even what your publisher requires. You can apply multiple licenses (dual licenses) as long as you have never granted an exclusive license, but this is usually encountered only with code and software as a means of circumventing license stacking. That occurs when there are multiple code snippets used in a project that have different licensing requirements.
There are two primary standard license groups for data. Creative Commons (CC) offers 6 licenses: CC-BY; CC BY-SA; CC BY-ND; CC BY-NC; CC BY-NC-SA; and CC BY-NC-ND. Open Data Commons (ODC) offers 2 licenses: ODC-By; and ODC-ODbL. There is also releasing the data to the Public Domain which still requires a license, but one that states that you relinquish all rights to the data. Both CC and ODC offer this option (see below). For software and code, there are many other licenses including the CC and ODC groups offerings.
Public Domain licenses, or waivers, are frequently suggested for open data. They rely on community norms for attribution, and they prevent the problem of attribution stacking, where a researcher uses data from multiple datasets and provides attribution for each component. Generally in the academic research world, attribution is freely given and is expected to be given. Not doing so could be grounds for plagiarism and considered research misconduct.
ODC also offers an open database license, the ODbL. Creative Commons offers a Public Domain Mark (PDM) for works no longer restricted by copyright. It acts like a tag or label to identify resources that are freely available for use by anyone.
Licenses for software, code, and packages. The Open Source Initiative (OSI) maintains a list of current licenses available for software and includes a FAQ section. These include the GNU GPL and LGPL licenses, the Apache license, BSD licenses, MIT license, Mozilla Public license, and the Eclipse license. They also provide information