Vilidy & Reliability in simple words

Validity refers to the degree to which a test measures what it is supposed to measure.
Forms of validity:
1) Content validity -> To what extent does this test represent the general domain of interest?
2) Criterion-related validity -> To what extent does this test correlate highly with another test?
3) Construct validity -> To what extent does this test reflect the construct it is intended to measure?
4) Consequential validity -> To what extent does this test create harmful consequences for the test taker?

Reliability is the degree to which a test consistency measures whatever it is measuring. The more reliable test is, the more confidence we can have that the scores obtained from the test are essentially the same scores that would be obtained if the test were readministered to the same test takers at another time or by a different person.
Types of reliability:
1) Stability -> Give one group the same test at two different times, and correlate the two scores.
2) Equivalence -> Give alternative test forms to a single group, and correlate the two scores.
3) Equivalence and stability -> Give two alternative tests to a group at two different times, correlate the scores.
4) Internal consistency -> Give test to one group, and apply split-half, Kuder-Richardson, or Cronbach’s alpha to estimate the internal consistency of the test items.
5) Scores -> Give copies of a set of tests to independent scores or a single score at different times, and correlate or compute the correlate or compute the percentage of score agreement

Reliability tells about the consistency of the scores produced; validity tells about the appropriateness of a test. Although a valid test is always reliable, a reliable test is not always valid. In other words, if a test is measuring what is supposed to be measuring, it will be reliable, but a reliable test can consistency measures the wrong thing and be invalid.

According to ‘Educational Research’, Ninth Edition by L.R. Gay, Geoffrey E. Mills & Peter Airasian