Reference style: what is it?

A reference style defines the rules you have to follow to refer to sources. There are many different reference styles, often linked to a certain field. Also certain scientific journals have their own reference style. Some examples of reference styles are: MLA, APA, Vancouver, IEEE, OSCOLA,...

Pointers

References have to be:

Complete (the reader needs to be able to find the same text by themself)
Consistent (use the same reference style throughout the text)
Simple (no excess information)
Legible (if you omit the reference, the text must still be readable)

Tools that help

There are several tools you can use to use a reference style correctly. One of them is citation software, such as EndNote, Mendeley, Zotero, and many others. You can see citation software as databases with your sources, where you pick a reference and drop it into your reference list. When used correctly, the softwares allows you to update reference styles automatically, which saves a lot of time.

You can also use a citation generator like Scribbr, that will format your sources for you in either APA or MLA.