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A general description is here.
It incorporates (slightly revised) copies of packages ‘multicore’ and ‘snow’ (excluding MPI, PVM and NWS clusters). Code written to use those packages should work unchanged (apart from changing any references to their namespaces to a reference to ‘parallel’).
It also contains support for multiple RNG streams following L'Ecuyer _et al_ (2002), with support for both ‘mclapply’ and ‘snow’ clusters.
(It is not yet complete: some enhancements are planned by R 2.14.0.)