Promoter:A DNA segment recognized and bound by RNA polymerase during initiation of transcription - it's like the sheet music telling our musicians when to start playing.
Operon:A functioning unit of genomic DNA containing several genes under the control of a single promoter - it's like different sections in our orchestra all following one set of instructions.
Epigenetics:Changes in organism caused by modification of gene expression rather than alteration of genetic code itself - this would be like changing the performance by adjusting lighting or acoustics, not altering the actual notes played.