This article can be a good item to go through.

Validity vs Reliablity

There are basically two aspects of validity: (i) internal validity- that should accurately be measured by right items or questions, for example you send an instrument or a questionnaire to the study participants where all items or questions will represent the study aims or interests that you want to measure for. (ii) External validity- that usually called generalizability, for example the findings from the study is applicable or generalizable to another context.

Similarly, reliability in two ways: (i) internal reliability- is a measure of how well your test is measured. It is also known as internal consistency and precision in measurement, for example to what extend the items or questions are consistently measured that have been used in the instrument or questionnaire. The most widely used statistic in order for checking internal reliability is Cronbach’s α. (ii) External reliability- means that your test or measure can be generalizable or repeatable, for example you are using an instrument so as to measure depression that should be able to detect depression in different age groups and socio-economic status.