A zapper is used to kill numerous parasites etc all in one go! It was developed by Hulda Clark but made better by others such as Don Croft with his Terminator II model. The Rife STYLE machines you talk about can kill parasites etc which the zapper finds hard. A Zapper can kill almost all parasites except it will take wearing it 24/7 and low frequency like a T2 to finally get the deep in gits. A rife style machine (F-Scan2) is used for these and can kill some problems in 1-3 mins. It is advisable to do more on a problem because of eggs and reoccurence. Its also down to budget so if you strapped for cash then get a Terminator II and wear it everyday as long as you or minimum 1 hour!
Alot of people use basic zapping before using their rife style machines. This method allows more concentratio with rife equipment on the left overs. If you were to knock everything out with basic fg's it would take ages inputing all the frequencys etc. So zap then Rife!