The PCR is used to amplify a single DNA or RNA molecule. The first use of PCR was in 1977 by Kary Mullis, who developed a method for amplifying DNA strands. Since then, it has been used to amplify nucleic acids from many different organisms.
The purpose of using PCR is to make more copies of the desired nucleic acid sequence. This can be done by using two DNA polymerase enzymes: one that will copy each strand (usually Taq polymerase) and another that will copy an entire strand at once (usually Pfu polymerase).
The Pfu enzyme can be designed so that it only makes single-stranded DNA or RNA molecules, while the Taq enzyme will make double-stranded molecules or single-stranded molecules with bulges in them.