The modern web runs on Representational State Transfer, or REST, allowing applications to consume services from a variety of "RESTful" services in order to sustain a distributed architecture. In other words, you don't have to own all the hardware, software, and data simply to build a web application nowadays.
Each REST service is typically served up via a REST endpoint, at a browsable URL. Think of these endpoints as oak trees in a large forest, each producing consistently large amounts of acorns, to the point where they are available on demand. As a web developer, much like a squirrel, with very little effort you are free to browse around and gather acorns from as many trees as you need in order to build up your cache of food for the winter - or the application you hang your hat on.
Of course, there are plenty of in-depth tutorials out there on REST. (This blog from Elkstein.org looks really well-done.) So, we'll keep these pages short, sweet, and focused on some common map-making techniques.