One of the most important concepts in Plasma is that of the "DataEngine". DataEngines are used to deliver data from somewhere, often from an internet service for example, to your applet. In this tutorial we'll discuss DataEngines, what they are, and how to use them from your Plasma applet.
Plasma supports many different DataEngines out of the box which deliver all sorts of varied information about the state of the machine, e.g. CPU usage, memory usage, position of the mouse pointer; to things such as the current weather report, current time or the latest comic from the internet.
Plasma comes with a handy tool called plasmaengineexplorer to list the running DataEngines and examine them to see what kind of data and fields them send. plasmaengineexplorer can be simply started from the shell as so.
TODO: finish tutorial.