Resistance:

The property of a material or conductor to oppose the flow of charge in a conductor is called resistance. The resistance R of a conductor is defined as the ratio of potential difference V across the conductor to the current I flowing through it. Therefore,

\(R=\frac VI\)

Or, V = IR

OR, \(I=\frac VR\)

In the SI system, the unit of R is the ohm (Ω), the unit of current is the ampere (A) and the unit of V is the volt (V)

Resistivity:

Experimentally, it is found that the resistance, R of a conductor is 

1. Directly proportional to the length, l of the conductor. i.e., R ∝ l ------- (i)
   
   
2. Inversely proportional to the area of the cross-section, A, of the conductor i.e.,
   
   \(R\propto\frac1A\) -------- (ii)

Combining relations (i) and (ii) we get, 

\(R\propto\frac lA\)

∴ \(R=\rho\frac lA\) -------- (iii)

Where, ρ is constant for the material of the conductor, called the resistivity of the material. When, l= 1m, A = 1m² then R=ρ. Hence, the resistivity of a material is the resistance of the material per unit length per unit cross-sectional area.

The unit of resistivity (ρ) is ohms meter (Ωm)

Click here for the derivation of effective resistance of a number of resistors connected in the series and parallel.