Reliability and validity are often confused, but the terms actually describe two completely different concepts, although they are often closely inter-related. This distinct difference is best summed up with an example:
Reliability is an essential component of validity but, on its own, is not a sufficient measure of validity. A test can be reliable but not valid, whereas a test cannot be valid yet unreliable.
Reliability, in simple terms, describes the repeatability and consistency of a test. Validity defines the strength of the final results and whether they can be regarded as accurately describing the real world.