Choosing the correct style for your purpose will be directed by your discipline, publisher or university. Schools at your university will provide a style guide outlining the style they utilise and the required format. There can be slight variations in format despite being the same style, so it is important to check even if you are using referencing software. Consistency is a key feature in correct referencing.
A number of styles exist in referencing. They usually fall into two categories:
Heart disease is Australia's biggest killer (Smith, 2010).... OR
Smith (2010) has shown heart disease is Australia's biggest killer.
Smith  has shown heart disease is Australia's biggest killer.
Some of the more popular referencing styles are:
Harvard referencing uses the Author/Date format for in text citations.
Author, AA Year, Title in italics, Publisher, Place of publication.
Author AA Year, 'Title of article', Title of Journal in italics, vol. #, no. #, pp. 1-2.
This page contains a sample guide from the University of Sydney. Please check with your university or publisher for their guidelines.
MLA Referencing follows the Author/page number format for in text citations.
Author, Firstname. Title of Book. Place of Publication: Publisher, Year of Publication. Medium of Publication (eg.Print)
Author, Firstname. "Title of Article." Title of Journal Volume. Issue (Year): pages. Medium of publication.
The Vancouver style of referencing uses the Author/Number system. A detailed guide of the Vancouver Style can be found in the e-book Citing Medicine.
Author AA. Title: subtitle. Edition (if not the first). Place of publication: Publisher; Year.
Author AA. Title of article. Title of journal, abbreviated. Date of Publication: Volume Number(Issue Number): Page Numbers.