updating overview to include two-process model

docs/source/overview.txt

@@ -15,9 +15,12 b' is somewhat limited for extended interactive use.'
 
 The goal of IPython is to create a comprehensive environment for
 interactive and exploratory computing.  To support this goal, IPython
-has two main components:
+has three main components:
 
 * An enhanced interactive Python shell.
+* A decoupled two-process communication model, which allows for multiple
+  clients to connect to a computation kernel, most notably the web-based
+  :ref:`notebook <htmlnotebook>`
 * An architecture for interactive parallel computing.
 
 All of IPython is open source (released under the revised BSD license).
@@ -172,11 +175,42 b' Main features of the interactive shell'
   prompts and whitespace) and uses doctest-compatible prompts and
   output, so you can use IPython sessions as doctest code.
 
+Decoupled two-process model
+==============================
+
+IPython has abstracted and extended the notion of a traditional
+*Read-Evaluate-Print Loop* (REPL) environment by decoupling the *evaluation*
+into its own process. We call this process a kernel: it receives execution
+instructions from clients and communicates the results back to them.
+
+This decoupling allows us to have several clients connected to the same
+kernel, and even allows clients and kernels to live on different machines.
+With the exclusion of the traditional single process terminal-based IPython
+(what you start if you run ``ipython`` without any subcommands), all
+other IPython machinery uses this two-process model. This includes ``ipython
+console``,  ``ipython qtconsole``, and ``ipython notebook``.
+
+As an example, this means that when you start ``ipython qtconsole``, you're
+really starting two processes, a kernel and a Qt-based client can send
+commands to and receive results from that kernel. If there is already a kernel
+running that you want to connect to, you can pass the  ``--existing`` flag
+which will skip initiating a new kernel and connect to the most recent kernel,
+instead. To connect to a specific kernel once you have several kernels
+running, use the ``%connect_info`` magic to get the unique connection file,
+which will be something like ``--existing kernel-19732.json`` but with
+different numbers which correspond to the Process ID of the kernel.
+
+You can read more about using :ref:`ipython qtconsole <qtconsole>`, and
+:ref:`ipython notebook <htmlnotebook>`. There is also a :ref:`message spec
+<messaging>` which documents the protocol for communication between kernels
+and clients.
+
+
 Interactive parallel computing
 ==============================
 
 Increasingly, parallel computer hardware, such as multicore CPUs, clusters and
-supercomputers, is becoming ubiquitous. Over the last 3 years, we have
+supercomputers, is becoming ubiquitous. Over the last 8 years, we have
 developed an architecture within IPython that allows such hardware to be used
 quickly and easily from Python. Moreover, this architecture is designed to
 support interactive and collaborative parallel computing.