The periodic amplitude modulation can perhaps best describe the oldest modulation effect (well known from old tube amps of Fender, Vox and other manufacturers). The only thing periodically changing here is the output volume, which can result in a rhythmic "chopping" effect in case of maximum setting. Stellar examples of utilizing the tremolo to its full extent are "How Soon is Now" by the Smiths or "Howlin' for You" by The Black Keys. The effect is usually implemented with a clean sound, it may sound pretty interesting with distortions as well though.
Speed and intensity can be tweaked, in some cases even the waveform used for cutting back the volume.