A Transcription factor is a protein that binds to specific DNA sequences and thereby controls the transcription of genetic information from DNA to mRNA. Transcription factors perform this function alone or with other proteins in a complex, by promoting (as an activator ), or blocking (as a repressor ) the recruitment of RNA polymerase (the enzyme which performs the transcription of genetic information from DNA to RNA) to specific genes.
In this way they coordinate the cell genes expression with external events.
Transcriptional Regulation of Metabolism 2006
General mechanisms of Nuclear Factor action - Cartoon