Transcription factors are DNA binding proteins which can repress or activate genes by binding to the promoter regions or other parts of the DNA and giving or denying access to transcriptional enzymes.
Transcription factors are often triggered by upstream events such as cell receptor/ligand interactions.
Transcription factors work to change gene expression by increasing or decreasing the transcription of particular gene. This in turn will lead to an increase or decrease of RNA molecule transcripts and theoretically a decrease in translated protein product.