The cminusminus domain is no longer valid (though it has more modern CSS), also it lacks links to all the informative papers!
C-- is very similar overall to LLVM IR, though there are crucial differences, but overall you could think of them as equivalent representations you can map between trivially (albeit thats glossing over some crucial details).
In fact, a few people have been mulling the idea of writing a LLVM IR frontend that would basically be a C-- variant. LLVM IR has a human readable format, but its not quite a programmer writable format!
C-- is also the final rep in the ghc compiler before code gen (ie the "native" backend, the llvm backend, and the unregisterized gcc C backend).
theres probably a few other things I could say, but that covers the basics. I'm also involved in GHC dev and have actually done a teeny bit of work on the c-- related bits of the compiler.
relatedly: i have a few toy C-- snippets you can compile and benchmark using GHC, in a talk I gave a few months ago https://bitbucket.org/carter/who-ya-gonna-call-talk-may-2013... https://vimeo.com/69025829
I should also add that C-- in GHC <= 7.6 doesn't have function arguments, but in GHC HEAD / 7.7 and soon 7.8, you can have nice function args in the C-- functions. See https://github.com/ghc/ghc/blob/master/rts/PrimOps.cmm for GHC HEAD examples, vs https://github.com/ghc/ghc/blob/ghc-7.6/rts/PrimOps.cmm for the old style.
The Manual: http://www.cs.tufts.edu/~nr/c--/extern/man2.pdf
The manual contains the specifications and a few code examples. It looks like it's easy to learn, but it's a little different than other languages.