Contact method:
This method is used when the body (whose temperature is to be measured) and the sensor(which is measuring the temperature) can remain in contact with each other, in other words, we can say that if the body and the sensor can remain in contact with each other during the measurement of temperature than contact method is used.
Non-contact method:
This method is used when the body (whose temperature is to be measured) and the sensor (which is measuring the temperature) are not allowed to remain in contact with each other, in other words, we can say that if the body and the sensor are not allowed to remain in contact with each other during the measurement of temperature then non-contact method is used.