Referencing and academic integrity
The American Psychological Association (APA) System
A secondary source is a work that you have only read about, and have not read the original document. For example, a study may be summarised in a journal article you are reading. The study itself is a primary source, whilst the journal article referring to it is the secondary source.
To refer to a secondary source, list the primary and secondary sources in the text, but only fully reference the secondary source you have actually read in your references page.
Ward and Green (2005, as cited in Phillips & Jones, 2008) found that...
It was found that... (Ward & Green, 2005, as cited in Phillips & Jones, 2008)
Phillips and Jones (2008) only would be mentioned in the References page.
You should always try and read primary sources!