Agree - read the zapper and related forums for the full-blown descriptions. SOTA's site also explains the difference between zapping and becking. The quick answer is -frequency, intensity, location of application, and intended results.
Hulda Clark 's first zapper output around 30,000 Hz, but since then she and others have modified devices to output at various frequencies, especially those many many different frequencies used for zapping specific pathogens (there is a long chart of these); the Croft zapper outputs 15.X Hz (fifteen point something, I forget exactly what the point something is). Croft uses a much lower frequency due to his observations that these provide many more favorable harmonic frequencies while also being close to the where the Schumann frequency is supposedly headed (up). Zappers in general are powered by simple transistor batteries that use no voltag-boosting circuitry, so you are getting around 9 volts intensity. The intended results are to kill the various parasites living in the relative shallow locations of the body.
Becking, or 'blood electrification' is using a low freqency of approximately 4 Hertz (which I believe is intended to be roughly half of where the Schumann frequency is.... or at least once was); the SOTA intrument I have (Silver Pulser) uses some voltage-boosting circuitry using the same 9 VDC transistor battery. Compared to zapping which can be applied to nearly any spot on the body, Becking is done only on pulse-points, primarily those at the wrists. The intended results are to kill the various parasites in the ciculatory/blood system.
I've done both and can attest to the noticable difference in feeling of intensity between the two; the Zapper is a slight tingle that can become annoying when done on sensitive skin surfaces; the Beck method is a very deep feeling pulse, not annoying at all, but if if turned up high enough, actually causes the muscles in hands to strongly contract.