Let's do this by commenting an example. It's from doing -ddump-ds on this code:

skip2 m = m : skip2 (m+2)

Before we jump in, a word about names of things. Within GHC, variables, type constructors, etc., are identified by their “Uniques.” These are of the form `letter' plus `number' (both loosely interpreted). The `letter' gives some idea of where the Unique came from; e.g., _ means “built-in type variable”; t means “from the typechecker”; s means “from the simplifier”; and so on. The `number' is printed fairly compactly in a `base-62' format, which everyone hates except me (WDP).

Remember, everything has a “Unique” and it is usually printed out when debugging, in some form or another. So here we go…

Desugared:
Main.skip2{-r1L6-} :: _forall_ a$_4 =>{{Num a$_4}} -> a$_4 -> [a$_4]

--# `r1L6' is the Unique for Main.skip2;
--# `_4' is the Unique for the type-variable (template) `a'
--# `{{Num a$_4}}' is a dictionary argument

_NI_

--# `_NI_' means "no (pragmatic) information" yet; it will later
--# evolve into the GHC_PRAGMA info that goes into interface files.

Main.skip2{-r1L6-} =
    /\ _4 -> \ d.Num.t4Gt ->
        let {
          {- CoRec -}
          +.t4Hg :: _4 -> _4 -> _4
          _NI_
          +.t4Hg = (+{-r3JH-} _4) d.Num.t4Gt

          fromInt.t4GS :: Int{-2i-} -> _4
          _NI_
          fromInt.t4GS = (fromInt{-r3JX-} _4) d.Num.t4Gt

--# The `+' class method (Unique: r3JH) selects the addition code
--# from a `Num' dictionary (now an explicit lambda'd argument).
--# Because Core is 2nd-order lambda-calculus, type applications
--# and lambdas (/\) are explicit.  So `+' is first applied to a
--# type (`_4'), then to a dictionary, yielding the actual addition
--# function that we will use subsequently...

--# We play the exact same game with the (non-standard) class method
--# `fromInt'.  Unsurprisingly, the type `Int' is wired into the
--# compiler.

          lit.t4Hb :: _4
          _NI_
          lit.t4Hb =
              let {
                ds.d4Qz :: Int{-2i-}
                _NI_
                ds.d4Qz = I#! 2#
              } in  fromInt.t4GS ds.d4Qz

--# `I# 2#' is just the literal Int `2'; it reflects the fact that
--# GHC defines `data Int = I# Int#', where Int# is the primitive
--# unboxed type.  (see relevant info about unboxed types elsewhere...)

--# The `!' after `I#' indicates that this is a *saturated*
--# application of the `I#' data constructor (i.e., not partially
--# applied).

          skip2.t3Ja :: _4 -> [_4]
          _NI_
          skip2.t3Ja =
              \ m.r1H4 ->
                  let { ds.d4QQ :: [_4]
                        _NI_
                        ds.d4QQ =
                    let {
                      ds.d4QY :: _4
                      _NI_
                      ds.d4QY = +.t4Hg m.r1H4 lit.t4Hb
                    } in  skip2.t3Ja ds.d4QY
                  } in
                  :! _4 m.r1H4 ds.d4QQ

          {- end CoRec -}
        } in  skip2.t3Ja

(“It's just a simple functional language” is an unregisterised trademark of Peyton Jones Enterprises, plc.)

The term "unregisterised" really means "compile via vanilla C", disabling some of the platform-specific tricks that GHC normally uses to make programs go faster. When compiling unregisterised, GHC simply generates a C file which is compiled via gcc.

Unregisterised compilation can be useful when porting GHC to a new machine, since it reduces the prerequisite tools to gcc, as, and ld and nothing more, and furthermore the amount of platform-specific code that needs to be written in order to get unregisterised compilation going is usually fairly small.

Unregisterised compilation cannot be selected at compile-time; you have to build GHC with the appropriate options set. Consult the GHC Building Guide for details.