What is an operon, and how does it regulate gene expression?

It's a set of is a set of genes regulated by the same promoter sequence, usually coordinating related cell functions.

They are transcribed together and codify a single string of messenger RNA, which can be lately trans-spliced ("cut apart") or translated together as well. In the first case, the mRNA is called monocistronic, meaning it'll codify only one gene product. When it translates into more than one product the mRNA is called polycistronic.

Here we have a schematic representation of a polycistronic operon structure with the enhancer/silencer, operator and promoter regions (regulatory sequences), ORFs (open reading frames, the bits that will be later transcribed into polypeptides), UTRs (untranslated regions, which are not related to specific aminoacids but may have important regulatory roles as well) and the further steps in gene expression: transcription - from DNA to mRNA - and translation - from mRNA to polypeptides.

Operons are mainly found in procaryotes, and one in special was the first one to be discovered and is the most well-known: the lactose operon, or lac operon, which coordinates lactose metabolism in the lack of glucose, cell's main energy source. It's been studied in model Escherichia coli, though other enteric (intestinal) bacteria also have it. Another quite relevant operon is trp, also found in E. coli and responsible for the production of the aminoacid tryptophan.