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.
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).