Resistance
Ohms Law
The current in an ohmic conductor is proportional to the voltage across it, provided that the conditions are kept constant
Voltage (V) V = R x I Resistance (Ω) Current (A)
Investigating Ohms Law
To plot a graph of the resistance for any component (doesn't have to be a resistor) include a variable resistor in the circuit to change the current going through enabling you to draw a graph.
V/I and I/V Graphs
The circuit gives sets of values which can be plotted for different components. There are two different graphs and you only need to know how a diode, an ohmic conductor and a lamp vary with their resistance.
An ohmic device creates a straight line
A lamp creates a curved graph because the conditions change as the filament heats up A diode only is highly resistant up to a certain positive voltage where it becomes unresistant. It remains highly resistant into the negative volts until it breaks down. This is because its designed to only let current flow in one direction. 

Resistivity
The intrinsic properties of a metal (in an exam use the equation when defining it)
Resistance (Ω) R= pL Resistivity (Ω/m) Length (m)
A Cross Sectional Area (m^2)
A Cross Sectional Area (m^2)
All copper has the same resistivity value but different sized copper wires will have different resistance