A citation is the way that you indicate that any piece of information (an idea, quote, image, argument, etc.) that you are using in your work didn’t come from you. This might seem a little counterintuitive, as your writing is your own work, right? Yes, but research and academic writing are grounded in the idea that you are building upon existing knowledge. Whenever you use existing knowledge, you need to give credit to the creator through a citation.
There are many different formats for citations but no matter what they look like, they all accomplish the same essential goals: they allow your reader to identify and find the sources you are using in your work.
Every citation has two parts: 1) the in-text citation, and 2) the reference entry.
The in-text portion quickly indicates to your reader that you are using someone else's work. The reference entry is where your reader can find out more about the thing you are citing. This will include the full bibliographic information so that readers can find your original source for themselves.
Why do you have to cite? You’ve likely been told you have to cite to avoid plagiarism. While this is true (more on that on this help guide's Plagiarism page), it helps to also think about building on the ideas of others and also giving credit where credit is due.
There are two metaphors that can help in thinking about this.
One is Burke's Parlor.
The other is a family tree.
In academic writing, you are participating in an ongoing conversation. Citing is the primary mechanism that allows you to participate, and not just talk to yourself in a vacuum. In order to be a part of the conversation, you have to connect your ideas to the ideas of others through citations.