The resistance of a component is defined as the ratio between V and I. Its unit is the ohm, where 
Ohm’s law states that, for metallic conductor kept at a constant temperature, the current in a wire is directly proportional to the p.d. across its ends.
Resistance increases with temperature because, as the temperature increases, the positive ions inside the wire have more internal energy and vibrate with greater amplitude about their mean positions. This increases the frequency of collisions between the charge carriers and positive ions, so the charge carriers do more work and transfer more energy as they travel through the wire.
I-V characteristics of components:
- Resistors: fixed resistors’ resistances are constant so the p.d. across the resistor is directly proportional to the current in it. Such resistors are ohmic conductors, have a constant resistance, and behave the same way regardless of polarity.
- Filament lamps: the filament lamp is a non-ohmic component as its resistance isn’t constant; it increases with p.d. (due to the filament heating up). The filament lamp still behaves the same way regardless of polarity.
- Diode and LEDs: these are non-ohmic components and their resistance isn’t constant. The resistance is very high up to a threshold p.d. after which it drops sharply for every small increase in p.d. Diodes’ behaviours depend on polarity – they only conduct when conventional current flows in a specific direction. LEDs act in the same way as diodes but emit light of a specific wavelength (which determines their threshold voltages).
LDRs (light dependent resistors) are components made of semiconductors whose density of charge carriers, and therefore resistances, depends on light intensity. In light conditions, the LDR’s density of charge carriers rises so its resistance falls.