The Library of Congress defines Primary Sources as follows:
Primary sources are the raw materials of history — original documents and objects which were created at the time under study.
They are different from secondary sources, accounts or interpretations of events created by someone without firsthand experience.
(http://www.loc.gov/teachers/usingprimarysources/)
For disciplines other than history, consider these examples of primary, and secondary, sources.
This chart illustrates the four 'V's that distinguish Big Data from Data.
For more in-depth discussion of Big Data, the four Vs, and much more, see the 2015 report NIST Big Data Interoperability Framework, Volume 1 Definitions (NIST = National Institute of Standards). Skim the table of contents to click through to sections of interest, for example Chapter 2 Big Data Definitions (page 4 of the document, page 12 of the pdf page numbering).