Gene expression refers to the control the cell has over when and how much protein is created from a particular gene. Every cell nucleus contains a complete copy of the organism's DNA, but only a small portion of the total genes are expressed, or translated into proteins in a particular cell. For example, a cell in a hair follicle may express a gene which creates keratin, or the protein that hair is made up of, but a gene in a nerve cell would not express that gene.
The degree to which a gene is expressed is controlled by a promoter. A promoter is a region or sequence in a cell's DNA which is located outside the coding portion of the gene. A promoter may be activated by a protein called a transcription factor. When a transcription factor interacts with the promoter of a particular gene, RNA polymerase, an enzyme that transcribes messenger RNA from DNA, binds to the DNA and positions itself to begin its work. The result of this transcription process, messenger RNA, is the informational input to the translation, or protein generation process.
Transcription factors where some of the first individual genes discovered. In the early days of gene discovery, drosophila, or fruit fly larvae were irradiated, and as a result some of them displayed mutations in their appearance, or phenotype. These mutations could be passed to subsequent generations, which showed that the change had been genetic, or a new genotype had been created. One prime example is the "Wingless" gene which is abbreviated wnt. Researchers discovered the the wingless/wnt pathway not only controls wing development in the fruit fly, but wnt family members are used to control development of virtually all organisms. Likewise, when the "hedgehog" gene was originally named because it's mutation lead to a drosophila phenotype of a small burr, or "hedgehog", but if you google "sonic hedgehog", you will see that this same pathway extends all the way up to mammals. Yes, the "sonic" portion of the name was inspired by the video game, but only after every other type of hedgehog was exhausted.
So how can we use our control of gene expression by promoters? With the rise of the biotech industry, researchers have become interested in the economical production of therapeutic proteins for the medical industry. Since creating proteins in a laboratory environment is so expensive, leading an extremely expensive product, where quality is difficult to control, why not just have the desired protein be put into a farm animals milk, and then milk the animal and purify the desired protein? This could be done by placing the desired gene for the transgenic product behind a promoter which is activated in the mammary gland of the host animal. This has been tried, developed and demonstrated by biotechnology companies interested in therapeutic protein production.