The sim configuration is a set of files
that makes up a very basic EMC2 using
ini, tbl, var, and a single hal file
for simulated limit switches.  It also
finds a file in configs/common that is
a core_sim.hal.  This file provides that
functionality in common with other
configurations.

In addition you can chose between various GUIs 
(Graphical User Interfaces), so you can find the 
one that you like.
Right now the possible choices are:

-- axis.ini (AXIS: a modern GL-based interface in python)

-- axis_9axis.ini (AXIS based, 9-axis machine: XYZABCUVW)

-- axis_mm.ini (AXIS based config, with mm settings)

-- keystick.ini (a terminal, curses based non-X interface)

-- lathe.ini (simulation for a lathe, AXIS-interface)

-- mini.ini (advanced tcl/tk interface)

-- servo_sim.ini (simulation for a servo machine)

-- tkemc.ini (another tcl/tk interface, lots of people like this one)

-- tracking-test.hal (a demonstration of hal_input and vismach. a neat toy)

-- tripod.ini (non-trivial kinematics machine)

-- xemc.ini (native X interface, runs ok on older machines)

Select one of these if you have no functional machine 
but want to edit some basic configuration stuff
or play with one of the EMC2 displays.

There is also a more complex simulated machine
called "servo_sim" that has full PID loops and
simulated motors and encoders.  This "machine"
can be used for more sophisticated testing 
and tuning.