docs: Outline the basic workflow. (#2974)

This short narative also helps introduce 'client' and 'context' terminology.

docs/intro/03-how-it-works.md

@@ -14,7 +14,28 @@ The server collects the results from all of the captured browsers and presents t
 
 This is only a very brief overview, as the internals of how Karma works aren't entirely necessary when using Karma.
 
-However, if you are interested in learning more, Karma itself originates from a university thesis, which goes into detail about the design
+## Outline of workflow.
+
+Here is roughly how Karma works:
+
+After starting up, Karma loads plugins and the configuration file, then starts its local web server which listens for connections.
+Any browser already waiting on websockets from the server will reconnect immediately. As part of loading the plugins, test reporters
+register for 'browser' events so they are ready for test results.
+
+Then karma launches zero, one, or more browsers, setting their start page the Karma server URL.
+
+When the browsers connect, Karma serves a 'client.html' page; when this page runs in the browser it connects back to the server via websockets.
+
+Once the server sees the websocket connection, it instructs the client -- over the websocket -- to execute tests.  The client page opens an iframe with a 'context.html' page from the server. The server generates this context.html page using the configuration. This page includes the test framework adapter, the code to be tested, and the test code.
+
+When the browser loads this context page, the onload event handler connects the context page to the client page via postMessage. The framework adapter is in charge at this point: it runs the test, reporting errors or success by messaging through the client page.
+
+Messages sent to the client page are forwarded through the websocket to the Karma server. The server re-dispatches these messages as 'browser' events.  The reporters listening to 'browser' events get the data; they may print it, save it to files, or forward the data to another service.
+Since the data is sent by the test framework adapter to the reporter, adapters and reporters almost always come in pairs, like karma-jasmine and karma-jasmine-reporter.  The detailed content of test-result data is of no concern to other parts of karma: only the reporter needs to know it format.
+
+Karma has many variations and options that may cause different workflow with different configurations.
+
+If you are interested in learning more about the design, Karma itself originates from a university thesis, which goes into detail about the design
 and implementation, and it is available to read [right here].
 
 [right here]: https://github.com/karma-runner/karma/raw/master/thesis.pdf