Polymerase Chain Reaction (PCR) is a key technique in molecular biology labs. It allows DNA to be copied many times over. Theoretically, each cycle doubles the number of DNA strands, and a typical procedure will have 30 cycles. So each strand of template DNA creates ~1 billion copies!
PCR exploits three concepts: DNA melting, primer annealing, and DNA polymerase 5' to 3' elongation. First the temperature is raised to around 100C, causing the DNA double helix to reversibly change from double stranded to single stranded. This allows the primers access to the base pairs. The temperature is lowered just enough for primers to bind to their target sequences. The sequence of the primers allows researchers to select what DNA sequences are copied. Once the primers are bound, the temperature is increased to allow DNA polymerase to start at the primer and copy DNA in the 5' to 3' direction.
All these temperature changes are done on a thermocycler. Typically these use the thermoelectric effect to maintain rapid temperature transitions and stable temperatures. Thermoelectric devices and thermocyclers are generally expensive. To bring biotech to the public, OpenPCR offers an open source thermocycler.