In prokaryotes, regulator genes often code for repressor proteins. Repressor proteins bind to operators or promoters, preventing RNA polymerase from transcribing RNA. Inducers cause repressor proteins to change shape or otherwise become unable to bind DNA, allowing RNA polymerase to continue transcription.
Other regulatory genes code for activator proteins. An activator binds to a site on the DNA molecule and causes an increase in transcription of a nearby gene. In prokaryotes, a well-known example of an activator protein is the catabolite activator protein or CAP, which is involved in positive control of the lac operon.
In eukaryotic gene regulation both activators and repressors are known to play important roles.