How do API work?

You're basically making "calls" to api specific address,and api specific terms or data request, and in return their server responds with the data you requested. You can than use that data to display it on your application.

For instance I used weather.com's weather API for test site. It sent it a request for the weather information I was looking for and the location of that weather, and their API sent back that up-to-date weather info as json, which I used PHP to parse, and than i had the ability to use php to provide that weather information in a way that matched my site.

Thanks Kevin Korte. I appreciate the response. James Barnett I was curious on how to create an API.

In addition to Kevin Korte's answer, if you want to read up more about what APIs are you can check out Make Use Of's article What Is An API & What Are They Good For?

Another good article about APIs http://readwrite.com/2013/09/19/api-defined#awesm=~oGOH4myC1KQW7i

Zapier also did a really good free course called "API Zero to Superhero". https://zapier.com/blog/free-api-course/