The type of transcription control in prokaryotes depends on the response of the operon in the absence of a specific regulatory protein.
In the case of negative control, a repressor protein inhibits the expression of genes in the operon. And the operon will be activated when the repressor is inactivated.
Example: lac operon.
In the case of positive control, operon will be activated in the presence of an activator. Thus, the operon will be turned off when the activator is absent.
Example: tryptophan operon.