Transcription is the synthesis of RNA from a DNA template. The primary transcript may then be modified or processed to the final product. Eventually, the RNA product is degraded to nucleotides. While all of these reactions are potential sites for control of gene expression, most control of gene expression is transcriptional. This is an example of a general principle of biochemical control: Pathways are controlled at the first committed step, and RNA synthesis is the first committed step of gene expression.
Different RNAs are transcribed with different efficiencies, so that structural (transfer and ribosomal) RNAs are transcribed very efficiently. Each transcription cycle leads to a single molecule. In contrast, one mRNA is translated into many protein molecules. This means that transcription of mRNA need not be so rapid.