Resistance depends on the geometry of a conductor as well as on what the conductor is made from, but it also depends on temperature (although we will often neglect this).

To understand the temperature dependence, consider a simple model of resistance. Electrons flowing through a conductor are impeded by atoms and molecules. The more these atoms and molecules bounce around, the harder it is for the electrons to get by. Thus, resistance generally increases with temperature.

For small temperature changes the resistivity varies linearly with temperature:

r = r_o (1 + a DT), where a is the temperature coefficient of resistivity.

We often write this in terms of resistance instead: R = R_o (1 + a DT)

which means we're assuming that length and area don't change as temperature changes. Generally the linear expansion coefficient is much less than the temperature coefficient of resistivity, which is why we can get away with this assumption.

In some materials (like silicon) the temperature coefficient of resistivity is negative, meaning the resistance goes down as temperature increases. In such materials an increase in temperature can free more charge carriers, which would be associated with an increase in current.

This can be exploited to make a resistor with a resistance that is almost independent of temperature. The resistor is made from two resistors placed in series. One resistor has a positive temperature coefficient, and the other has a negative temperature coefficient. The resistance values are chosen so that when the temperature changes, the increase in resistance experienced by one resistor is offset by the decrease in resistance experienced by the other.