When electrons move through a conductor, there's a chance that they will bump into the large stationary atoms and particles inside it, temporarily restricting its movement and creating a zone where more electrons are lumped together.
The bumps that make electron flow harder give the conductor its Resistance.
Each material has a characteristic resistance to the flow of electrons. In general, the more free electrons the material has, the less resistance it will have. Materials such as the metals copper, silver and gold have many free electrons in their structure, and are very good conductors of current. Other materials such as carbon, have less free electrons, but still enough to allow considerable current to flow.
The fact that carbon is very easy to mold into a particular shape make it very useful in the making of resistive elements for electronic circuits, since the thickness and length of the material help determine the end resistance of the component, parameters easily changed to create specific resistance values.
