The Harvard citation style first emerged in the 1880s at, no surprises, Harvard University. It is widely used in many different disciplines. It is called an author-date system because the citation includes the author of the work and the date in which the author wrote it. A reference list then appears at the end of your work to list all the citations that you have used.
A slight complexity to the style is that you can create citations in two ways. The difference depends on the context of your citation: whether you want to foreground another's words in your work, or whether you want to use their words as part of your own reasoning.
The location of the brackets makes all the difference.
1. Using the citation to introduce directly what another person thinks (this method is a strong 'look, this person says this')
2. Using the citation to discuss what others have said (this method merges the words of others more naturally into your work)
No matter the method you use, you will then need to create an alphabetically reference list of all the citations used:
Don't worry if this seems a little confusing; Cite Them Right provides all the answers you need. It shows you how to use the Harvard style for lots of different reference types: including books, book chapters, journal articles, and more!