The difference between Rife and ParaZapper is extensive but not too much so.
Rife provides specific frequencies to achieve various needs. Most Rife machines have a frequency generator that allows you to select certain frequencies and many of them use a device such as a plasma tube to transmit the signal. These are very expensive. A Rife unit costs from about $2500 US and up. Be very wary of the "Digital" series as they have technical limitations that they fail to mention.
ParaZapper also provides Rife frequencies buy provides them in preset modes which makes it simple and easy to use. The only models that provide the accuracy called for by Rife are the ParaZapper MY and the ParaZapper MY+Plus.