Resistance of a conductor is defined as the property by virtue of which the conductor opposes the flow of charge through it. It is represented as  or 

we have,

From ohm's law, 

V = IR

Hence, the resistance of a conductor can also be defined as the P.D. developed across it for unit current passing through it.

Unit of resistance: In SI, V is expressed in volts(V)and I is expressed in Ampere(A), hence the

resistance is expressed in V/A. this is called Ohm(Ω). 1 Ohm=1V/A.

A conductor is said to have one Ohm of resistance if the current through it is one ampere when a potential difference of one volt is applied across it.

Causes of Resistance: When current flows through a conductor, the free electrons frequently collide with the atoms on lattice sites of the conductor. These collisions slow down the motion of electrons and hence increase the resistance of the conductor. The resistance depends upon the no of collisions that the electrons make with the atoms in the wire.

Therefore, the resistance of a conductor depends upon the arrangement of the atoms in the conductor.