PCR
PCR consists of three steps (address is //esg.www.mit.edu:8001/esgbio/rdna/pcr.html):
1. Denaturation: you heat the DNA you want to sequence (template). This will make the double stranded DNA single-stranded.
2. Annealing: You decrease the temperature in the presence of short pieces of DNA (oligonucleotides or primers) that border the area of the genome you want to sequence. The decrease in temperature causes the bonds to form again or reanneal. The oligonucleotides, being present in large concentration, will bind first, forming many double-stranded/single-stranded junctions.
3. Extension: you once again increase the temperature, and in the presence of a special polymerase that works at higher temperatures, complementary strands of template are synthesized at the junctions.
These three steps are repeated over and over again, and each time the concentration of the strands of DNA you are trying to synthesize increases.