Please let me know what is dry bulb and wet bulb temperature and how it is measured?

It's the tried and true method of measuring relative humidity... Bulb refers to the bulb of a thermometer. There was a dual thermometer device, one with a cloth sock over the bulb you would wet and spin. Depending on how moist or dry the air was there will be a difference between the two. You referred to a chart to get the % relative humidity.