Resistance happens because the moving electrons collide with the ions in the material. In a fixed resistor, and in a metal wire at constant temperature, the resistance is constant, so the current is proportional to the p.d.
In a filament lamp the current heats the thin wire. The ions vibrate more, so there are more collisions with electrons and the resistance increases as the current increases. A diode is different: it has a very low resistance in the forward direction, so current flows easily, but a very high resistance in the reverse direction, so almost no current flows.
The resistance of an NTC thermistor decreases as its temperature rises. More current then flows for the same p.d. The resistance of an LDR decreases as the light intensity increases. Thermistors and LDRs are useful because their resistance depends on their surroundings. A diode lets current flow in one direction only.
