Sometimes you read about a potentially relevant work in another work. For example, a book may be discussed in a journal article you are reading. The book is the original (primary) source; the journal article citing it is the secondary source.
Avoid citing secondary sources if possible. Only works that you have actually read should appear in your reference list. If possible, as a matter of good scholarly practice, find the primary source, read it, and cite it directly rather than citing a secondary source.
However, if you are unable to get hold of the original source - e.g., if it concerns an out-of-print book, an older printed report, or a work not available in English - you may cite the secondary source.
When citing a secondary source:
If the year of publication of the primary source is known, also include it in the text citation.