What is the difference between a lab and field experiment?

A lab experiment is conducted in a well-controlled environment - not necessarily a laboratory - and therefore accurate and objective measurements are possible. The researchers decides where the experiment will take place, at what time, with which participants, in what circumstances and using a standardised procedure. On the other hand, field experiments are conducted in the everyday (i.e. natural) environment of the participants but the situations are still artificially set up. The experimenter still manipulates the IV, but in a real-life setting (so cannot control extraneous variables).