APIs are everywhere and you use them everyday. In the world of marketing APIs are the way applications communicate with each other over the web. This might be Zapier communicating with your email and Google Sheets, or your website communicating with Mailchimp whenever someone signs up for a newsletter on your website. Typically when you are using an API you will need to get a key from the app that you want to communicate with. This key lets the program you are using to connect, verify that you have the correct permissions and access to the data you want to utilize.