FYI, sparks are different from threads. Haskell has "forkIO" threads which are comparable to Erlang processes/go-routines (actually, they're cheaper than both since haskell allocates less stack for threads). Threads are what you want for concurrent programming (i.e., non-deterministic interleaving of behaviour, which with GHC can be run in either single-threaded or multi-threaded mode).

Sparks are for deterministic parallelism, the idea behind sparks is that you create a pool of sparks, each of which represent a small, finite bit of work that can be done in parallel.

In general sparks will represent a finite, deterministic amount of work, whereas threads represent a (potentially) infinite amount of non-deterministic work.