Data management is the practice of organizing and describing your research data in such a way that another researcher will be able to understand it. Good data management practice makes it easier to share and re-use data, leveraging the small pool of available funding so data are not lost but made available for future uses, such as enabling other researchers to replicate results or employing the data in new ways that could not be envisioned when they were first collected. Data security and study participant protections must still be considered.
A data management plan is a formal document that outlines what you will do with your data during and after you complete your research. This includes policies for access, sharing, and re-use, as well as preservation of data.
Many agencies across a range of disciplines now require a data management plan as part of funding proposals. See the Agency Requirements tab of UNC Libraries' Research Data Toolkit for various lists. More and more agencies are implementing requirements for DMPs, and funding agency data policies & guidelines vary. Be sure to check the proposal guidelines for the specific agency to which you are applying. Several address data sharing.