The term “lie detector” most commonly refers to a polygraph.
A polygraph machine records several physiological responses by a person who is being questioned. Those things mainly measured are perspiration, heart rate, breathing and blood pressure. The polygraph prints these responses on to a graph for an examiner to interpret.
When a polygraph is being administered, the examiner will usually ask a combination of irrelevant and relevant questions. Then based on the graph the machine produces, the examiner can identify if there has been any physiological response in the subject, and whether there has been any significant change to when answering the irrelevant and relevant questions. The assumption is that deception will, due to the stress induced by lying, lead to increased heart rate, sweating, and so forth.
However the reality is there can be a number of factors that can affect the readings detected by a polygraph – like for example nervousness in what is usually a high stake situation. For that reason polygraphs are not used in Australian Courts, because there is no consensus that polygraph evidence is reliable.