Slightly less in brief:
The genome – the DNA that holds all the instructions for building an organism – doesn’t actually do anything. Proteins do most of the work in a cell. Proteins of all shapes and sizes, along with a few other types of molecule, are the cellular machinery.
The genome is just a blueprint for this machinery. Many short sections of the genome – the genes – describe how to build proteins. Certain proteins read genes and make temporary copies, which other bits of cellular machinery then use to build new proteins.
So which genes get read?