What is the difference between validity and reliability?

Validity refers to the extent to which a test measures what it sets out to measure. For example, a study may have strong internal validity if there is a causal relationship between the independent variable and dependent variable. It can be assessed by looking at wether adequate controls have been used / counterbalancing / controlling extraneous variables / standardisation etc. On the other hand external validity refers to the extent to which the results can be generalised to other settings, people or time periods. Assessment may involve whether the study has a generalisable sample and the setting in which it was carried out. 

In research, reliability relates to the consistency of a particular study or test. For example, how easily a test can replicate the same results. It is a necessary, but certainly not sufficient condition for validity. (The relationship between these two constructs is where most students lack understanding). Again, it can be split into internal and external types. Where split-half methods would assess internal reliability, test re-test and inter-rater reliability would measure / allow one to assess the external reliability of any given test.