The measurement is based on variation of resistance from metallic wires (resistors) against variations of temperature. Materials most often used are platinum and nickel. Platinum offers a large temperature range and a good linearity. His pureness and chemical inertia guarantee a high stability of sensor itself. 

Relation between platinum resistance and temperature according Standard CEI 751 is as following formula (restricted):

                                                           Rt = Ro [ 1 + At + Bt² + Ct³ (t - 100)] 

R_t = Thermometer resistance at temperature t
R_o = Thermometer resistance at 0°C
t = Temperature in °C
A B C = Coefficients according to previous calibration ( C = 0 for positive temperature in °C)

Industrial probes and boards are base on:

R_o = 100 Ω
R 100°C = 138.5 Ω

FRANCE NFC 42330
GERMANY DIN 43760
GREAT BRITAIN BS 1904
INTERNATIONAL CEI 751

Acceptance ttolerances in °C:

Class A: ± (0.15 +0.002)
Class B: ± (0.3  +0.005)
is temperature value in °C