Reliability in research relates to accuracy or consistency of the data or the instrument used to measure or collect the data. This is acheived usually through repeated administration of the same instrument /tool to collect the data.
Validity refers to the instrument measuring what it is supposed to measure. It would have been tested in pilot studies.
eg using a thermometer ( a valid tool) to measure someones temperature is a reliabale way of doing so but if the person reading the thermometer makes an error or it isnt used correctly then it is unreliable.
And to continue from rosyciders 'reliable but not valid' example a 'valid but not reliabe' example is that if you have a clock that's stopped, then it's 'valid' i.e accurate two times a day, but it's not reliable to keep time.
Also, Scylax is so right, as a Psychology student the 'Gross' book should be your bible!