What is the difference between reliability and validity in psychological testing?

Reliability is the consistency of a measurement. A reliable test produces similar results when administered repeatedly to the same person or scored by different evaluators. Types include test-retest reliability measuring consistency over time and inter-rater reliability measuring consistency across scorers. Validity is whether a test measures what it claims to measure. Content validity means the test covers the full range of the concept. Criterion validity means the test predicts real-world outcomes. Construct validity means the test measures the theoretical concept it targets.