In this tutorial I will cover how to use clsource's REST Helper classes to create a RESTful API endpoint within a PW-powered site and how to connect to it from the outside world with a REST client. This is a quite lengthy tutorial. If you follow all along and make it through to the end, you should get both, a working REST API with ProcessWire and hopefully some more basic understanding how these APIs work. As always with PW, there are many ways you could do this. My way of implementing it and t