PCR is a technique used to amplify a specific DNA sequence. It involves repeated cycles of heating and cooling of the DNA sample to denature and separate the DNA strands, followed by primer annealing and polymerase extension. This results in exponential amplification of the target DNA sequence. PCR requires a DNA template, DNA polymerase, primers, nucleotides, and repeated cycling between high and low temperatures. It has applications in research, forensics, medicine and molecular biology.