A lab technique used to purify a substance, to remove a solvent (a liquid that a substance is dissolved in) from the substance, or to separate two or more components in a liquid mixture. Ideally, this is done by taking advantage of the fact that the different chemicals have different boiling points. The temperature is raised so the chemical with the lowest boiling point boils first, its vapor is shunted out of the flask and towards a different container, and then it is cooled so that it condenses back into a (more purified) liquid. The temperature can then be raised to boil the next chemical, etc., until only the chemical with the highest boiling point remains in the original container.
Selected distillation links: