What is the difference between reliability and validity

These are two concepts that are often confused in Psychology students of all ages. Reliability is simply whether your results can be replicated by someone else. In terms of an experiment this means that you need to have the same conditions for each member of the sample and use the same equipment. An example of reliability is inter-rater reliability, where different researchers observing behaviour will rate the behaviour in the same way.
On the other hand, validity is how "true" our results are. This means that a valid study is measuring what we want it to measure. An example of this is ecological validity, which means that our experiment is measuring the same behaviours as participants would show in the real world and their behaviour is not altered by being in a new setting. This also includes demand characteristics where a participant changes their behaviour based on what they think the experimenter wants making the experiment invalid.