A license communicates to prospective users what conditions you would like them to follow should they want to use the data, including how you would like the data to be cited.

Available options include:

• The Open Data Commons Open Database License (ODbL); recommended
• CC0 rights waiver, public domain equivalent; recommended
• Creative Commons licenses (primarily intended for creative works like art, writing, etc.)
• Apache or MIT licenses (primarily intended for software code; not so relevant for data)
• Writing one's own license (not recommended)

Choice of license may also be constrained by the repository where you deposit the data. Dryad, for example, enforces a CC0 rights waiver on all submissions.