What is the difference between reliability and validity?

Reliability is about whether the same result would be found if the experiment was repeated. An experiment must be replicable if it is to be reliable. One way to remember this is 'reliability requires replicability'.

On the other hand. validity is about whether the experiment is measuring what it says it is measuring. You can have a test that is very reliable, which suggests good things about the experiment, but the test may not actually have validity - it may be testing a completely different pattern/relationship.