The first step in PCR is to heat the sample so that the double-stranded DNA separates into two single strands - this is called denaturation. Then primers, short samples of DNA that match up to the ends of the DNA sequence of interest, are combined with the sample DNA. After this, a DNA polymerase is used to start DNA replication at the primer location. Finally the DNA is heated to separate the strands once more, and the whole PCR process begins again. The amount of the DNA segment of interest present in the sample increases exponentially with each PCR cycle: one copy becomes two, then becomes four, then becomes eight, etc; so generally, only 20 to 40 cycles are needed to determine if the DNA in question is present (and, if it is, to provide a sufficient sample for analysis).
All the steps of a polymerase chain reaction -- denaturing the DNA, applying the primers, and elongating the DNA -- happen at different temperatures, so after the initial mixture is put together, the steps can be controlled through a process known as thermocycling, in which the temperature is held at the necessary levels for just long enough for each step to take place. Thus, PCR is an efficient way of amplifying the amount of target DNA in a single test tube with little need for human intervention.
Polymerase chain reaction represented a revolution in biological technique when it was first developed in the early 1980s, and PCR's creator, Kary Mullis, won the Nobel Prize for Chemistry for his work in 1993.