RTD's resistance to temperature
There is a nice discussion of resistance probes here. 

These probes use a length of fine metal wound into a coil or formed onto a ceramic piece. These use the well known characteristic of the metal to change resistance over a temperature range. The PT type probes use platinum that is very fine and the probes are relatively expensive and fragile. None the less they are prized for their stability, accuracy and repeatability. 

The probes leverage the significant characteristic in that the metal resistance is a linear change with temperature over a usable range for a particular sensor. The resistance versus temperature characteristic of a particular probe will be determined as the ratio as follows:

Given:

R0 = resistance at 0°C

R100 = resistance at 100°C

Then:

Alpha = (R100 - R0) / (100 * R0)

The Alpha value for pure platinum has a value of .003925 ohm/ohm/°C. The Alpha value for the probes that you would procure would be specified by the manufacturer. They will vary because the chemical composition of the platinum is carefully adjusted to give the probe a certain resistance at a given temperature. Knowing the Alpha for your probe and the resistance at a particular temperature it is possible to determine the temperature of the probe at a measured resistance by using the formula above. 

Note that in reality nothing is ever so perfect and so it is normally necessary to apply calibration process with each probe to capture the resistance at two or more temperature points. This data can then be stored in a non-volatile way and then later retrieved to compute temperature when a resistance reading is made. This will lead to the highest accuracy readings. If more than two calibration points are used to increase accuracy the equivalent Alpha is computed for each span range between the calibration points and measured resistance is matched to the proper range before computing the actual temperature. There are times of course where the ultimate accuracy is not needed and the published Alpha value for the sensor is used without calibration.