What is the difference between reliability and validity?

Reliability refers to how consistent the results of a study are or the consistent results of a measuring test.

This can be split into internal and external reliability. Internal reliability refers to how consistent the measure is within itself. A personality test should produce the same results every time for the same participant. 

External reliability refers to how consistent the results are when the same procedures are carried out for a test. For example, if a research study takes place, the results should be almost replicated if the study is replicated. 

Validity refers to whether the study or measuring test is measuring what is claims to measure. Internal validity refers to whether it is exclusively the independent variable causing the change or whether there are confounding variables. External validty refers to how well the laboratory study can be generalised to real life settings. 

Remember, something can have reliability but not validity and validity but not reliability.