WordPress has numerous APIs (application programming interfaces): intentionally designed "ways you do" specific things in the software. Without mentioning the term much, previous sections have been all about WordPress's APIs; the hooks system is an API, for example, as are the functions for working with custom fields.
This section continues by taking you through a number of very important and bordering-on-advanced WordPress APIs. Learning these APIs will greatly empower you as a developer, and prevent you from doing things the long, hard, and wrong way.
To understand intuitively what an "API" itself is, think about a light switch. A light switch is an API for your home's electrical wiring: it's a simple interface that folks can use to control the behavior of something quite complicated (the wiring itself). You don't need to know as much about the wiring if you've got the switch, and that's the value of an API.
The first two chapters in this section present important APIs for s...