PCR (Polymerase Chain Reaction) is a laboratory technique used to produce many copies of a specific DNA region. 

Primers are short, single strands of RNA or DNA. They are usually about 18-22 bases.

Primers act as a starting point for the PCR process as DNA polymerase is unable to initiate synthesis by itself using just a single stranded piece of DNA. To synthesize DNA, the enzyme polymerase requires a bit of already built DNA or RNA that it can extend. In PCR, primers are used to hybridize with the sample DNA and define the specific region of DNA to be amplified. Polymerase can then extend this region by adding new nucleotides.