Promoter (genetics)

In genetics, a promoter is a section of DNA which starts the transcription of a gene. 
Promoters are near the genes they transcribe. They are on the same strand of DNA and are 'upstream'.
Promoters can be about 100–1000 base pairs long.
Overview.
For the transcription to take place, the enzyme that makes RNA, known as RNA polymerase, must attach to the DNA near a gene. 
Promoters contain specific DNA sequences which give the RNA polymerase a place to bind. Other proteins also help this to happen. Some can also stop it from happening. The whole thing is called "the regulation of gene expression". 
Promoters are important parts of the DNA. They work with other regulatory regions. Together, they adjust the level of transcription of a gene. So, genes get switched on when they are needed, and switched off when they are not. When they are on, they get adjusted up or down as needed.