Open data is data that anyone is free to use, reuse and redistribute without restriction (except, perhaps the requirements to attribute and sharealike). For a definition of open data see http://opendefinition.org/.
The Open Knowledge Foundation have a guide to open data and a more detailed open data handbook.
The benefits of sharing your research data include increased citations and impact, facilitation of collaboration and answering new research questions. Also, data sharing is now mandated by many research funders.
Making your data fair
Following the FAIR principles will ensure your data are Findable, Accessible, Interoperable and Re-useable.
What data do I need to keep?
It is not realistic, or desirable, to keep all research data for all time. Efforts to preserve data should be focused on the data which are likely to generate the most value over the long term.
The DCC has produced a guide to help researchers decide what data to keep.