What are some good places for me to learn more about API's?

I recently had an interview where I was asked to connect to th开发者_如何学运维e companies API and download a certain number of files onto my computer. I'm a student who's fairly new to programming, and I had no clue what an API was, let alone connect to one.

My Questions are:

What is an API?

and

What are some resources for me to learn how to work with API's? (I use PHP).

Thanks so much!

Lets say that I'm a company and I have products. I want to allow other people to display my products on their web site. How can I do this without giving them my database username and password and leaving a note that says "Please don't delete anything." ?

For brevity I could write a function/method that someone could invoke that returns a list of products. This method will ask that it is given some information, such as a category name. I could then write a separate method for add product if I wanted to allow it.

In other words, an API is an interface in which you can work with someone elses application in a secure manner. They control what you can and cannot do.