We've spent some time looking at how to send requests, but many users would argue that receiving requests is just as important, if not more so. Let's take a moment to understand the Salt API.
Very simply, the Salt API is a REST interface wrapped around Salt. But that doesn't tell you the whole story. The salt command is really just a command-line interface for Salt. In fact, each of the other Salt commands (salt-call, salt-cloud, and so on) is really just a way to access various parts of Salt from the command line.
The Salt API provides a way to access Salt from a different interface: HTTP (or HTTPS, preferably). Because web protocols are so ubiquitous, the Salt API allows software, written in any language that has the capability of interacting with web servers, to take advantage of it.