Secondary sources analyze, discuss, and comment upon primary sources. They provide a second-hand account and are one step removed from the original. Tertiary sources, on the other hand, are based entirely on secondary sources.
The Online Dictionary for Library and Information Science by Joan Reitz provides the following definition:
secondary source
- Any published or unpublished work that is one step removed from the original source, usually describing, summarizing, analyzing, evaluating, derived from, or based on primary source materials, for example, a review, critical analysis, second-person account, or biographical or historical study. Also refers to material other than primary sources used in the preparation of a written work. Compare with tertiary source.