Q.What are the different scales of temperature? What is the relation between them?

Thermometry -- the science of measuring temperature -- rests on the zeroth law of thermodynamics: if two systems are each separately in thermal equilibrium with a third system, they are also in thermal equilibrium with each other. This justifies using a single instrument, a thermometer, as that "third system," brought briefly into contact with a body until equilibrium is reached and then read off. A temperature scale is built by choosing two easily reproducible fixed points -- classically the ice point (melting point of ice) and the steam point (boiling point of water), both at one standard atmosphere -- assigning them reference numbers, and dividing the interval between into equal degrees; the Celsius scale (0 to 100) and Fahrenheit scale (32 to 212) are the two everyday examples, related by $(T_F-32)/180 = T_C/100$. More generally, any thermometer exploits some thermometric property (length of a liquid column, electrical resistance, gas pressure at fixed volume, EMF of a junction) that varies smoothly and monotonically with temperature, via the formula $T = [(P_T-P_1)/(P_2-P_1)]\times100$. Different thermometer types -- liquid-in-glass, constant-volume gas, resistance, thermocouple, thermistor -- trade off accuracy, range, sensitivity, reproducibility and response speed differently, so the right choice depends on the application.