In the context of genetics, a transcription factor is a regulatory protein that initiates the transcription of certain genes upon binding with DNA.
The binding of a transcription factor to a specific DNA sequence can result in either an increased rate of transcription of the gene, known as activated transcription, or repression of transcription, known as "silencing".
Any given gene will typically have its own pattern of binding sites for transcriptional activators and repressors ensuring that the gene is only transcribed in the proper cell type(s) and at the proper time during development.