If you use the texts, ideas, research data, pictures or any form of information published by somebody else, you must always refer to the source (the original publication), no matter where it originates from. For example your source may be an article from a scholarly journal, a thesis, a conference proceeding or a publication on the internet.
Referring to your sources correctly and in the right places increases the readability and verifiability of your publications.
A reference to a journal article usually contains:
Author(s), title of the article, title of the journal, year of publication, volume and/or issue and pages.
A reference to a book usually contains:
Author(s), title of the book, year of publication, (sometimes) place of publication and the name of the publisher.
A reference to an article/chapter in an edited volume (every chapter written by another author, book itself edited by one or more persons) contains both data on the specific chapter and on the book as a whole. The details of the specific chapter are stated first:
Author(s) of the chapter, year of publication, title of the chapter, the word "in" followed by the details of the book as a whole (including editors) and finally the page numbers of the chapter.
A reference to an online publication will, in addition to the information mentioned above, contain a reference to the location where the source can be found, for instance a URL (Uniform Resource Locator) or a DOI (Digital Object Identifier).