The sources of information are divided into three categories: Primary, Secondary & Tertiary. All three stages have different roles in presenting the information.
Primary sources – Scholarly journal articles, letters, conference proceedings, autobiographies, speeches, poems, interviews, etc.
Secondary sources – Textbooks, biographies, commentaries, reviews of law and legislation, etc.
Tertiary sources – Dictionaries/encyclopedias, Wikipedia, directories, manuals, textbooks, abstracting sources, etc.