A PCR machine, also known as a thermal cycler, is a DNA amplifier that regulates temperature and amplifies segments of DNA via the polymerase chain reaction. As Wikipedia points out, PCR machines are also sometimes used to facilitate reactions regarding enzyme digestion or rapid diagnostics.
Tubes holding reaction mixtures are inserted into a thermal block inside the PCR machine. The machine raises and lowers the block's temperature in discrete steps. PCR machines commonly cycle from 4 to 99 degrees Celsius, but some are able to cool as low as -3 degrees Celsius. Some PCR machines contain multiple thermal blocks so that several PCR reactions can be carried out at the same time, and some allow different temperatures in different sections of the thermal block.