diff between transformer and flowstep

Hi to all
can i know the actual diff between tranformer and flow service or flow step.
suppose : appendToDocList, date ,string ,math operations can apply on document by writing the series of transformers in a MAP step .but the same functionality we can write in flowservice also .then what it makes the exact diff between these transformer and flowservice.

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For your question, this thread has enough answer.
http://www.wmusers.com/forum/showthread.php?t=13019