If you are writing a simple essay (maybe as an English language learner) you probably did a little research to get a few ideas and maybe one or two quotes to support your argument. You probably didn't use any primary sourcesSee the glossary definition - perhaps you just found a couple of simple newspaper articles. For a simple essay that might be fine and you have satisfied the requirements of the assignment. However, a serious research report requires an understanding of the types of sources you use and an ability to evaluate those sources. You need to distinguish between primary, secondary, and tertiary sources.
Primary Sources are published accounts of original work. They are usually published in academic journals which deal with a particular field. In the scientific field they are usually accounts of experiments conducted by the researcher and her team. Many of these journals are very specialised. For example, a microbiologist specialising in food research might publish her results in the International Journal of Food Microbiology.
Primary Sources are often difficult to read and understand for the non specialist. If you are writing a report or a thesis in your own specialist area, you would be expected to cite mainly primary sources. Secondary sources are often useful for getting an overview of a particular topic.
If you are a historian, then primary sources mean any original text or historical object from the period under study. If you are a lawyer, primary sources are are original sources of law such as Acts of Parliament, whereas secondary sources are books, journals and commentaries.
Secondary Sources recount information from other sources, both primary and secondary. Textbooks are examples of secondary sources. There are also many journals aimed at interested non-specialists which give accounts of original research written written in simpler language. New ScientistGo to the New Scientist page is an example.
Tertiary Sources are indexes of already publishes sources which provide little or no extra information which cannot be found in those (primary or secondary) sources. Tertiary sources are mainly dictionaries, literature reviewsSee the glossary definition, bibliographies, and encyclopedias. They are useful for locating authoritative sources or for getting and overview of a particular subject matter. Citations from tertiary sources are not normally acceptable in academic articles.
Wikipedia is a wonderful resource. It is a tertiary source, much loved by students. However, it is not a reliable or authoritative source to be cited in any essay or report (unless you happen to be writing about Wikipedia itself). Use it as a research tool to find information and links to more authoritative sources, but don't cite it in your work.