An operon is a cluster of genes that are expressed simultaneously. They are usually a set of genes that code for proteins involved in the same metabolic process.
The lac operon codes for three genes that are involved in the breakdown of lactose.
If there is a high concentration of lactose in the cell, the cell is able to simultaneously express all these genes.
Gene expression is induced at a promoter region. Only a single promoter region needs to be activated in order to express all genes in an operon.
This is far more efficient than expressing each gene individually using different promoters.
The operon therefore represents a highly economical and effective way of organising a genome.
Operons are found almost exclusively in prokaryotes.