Outline
Resistance

Resistance, property of any object or substance of resisting or opposing the flow of an electrical current.
The quantity of resistance in an electric circuit determines the amount of current flowing in the circuit for any given voltage applied to the circuit, according to Ohm's law.

S.I. Unit

The commonly used unit of resistance is the ohm, which is the resistance of a conductor in which a potential difference of 1 volt causes a current flow of 1 ampere.

Symbol

The standard abbreviation for electric resistance is R and the symbol for ohms in electric circuits is the Greek letter omega[W].
For certain electrical calculations it is convenient to employ the reciprocal of resistance, 1/R, which is termed conductance, G. The unit of conductance is the mho (ohm spelled backwards) and the symbol is an inverted omega.

Resistivity

The resistance of an object is determined by a property of the substance of which it is composed, known as the resistivity, and by the length and cross-sectional area of the object, and by the temperature.

 fig.1
 fig.2 - Formula

At a given temperature, the resistance is proportional to the object's resistivity and length, and inversely proportional to its cross-sectional area.

Resistivity is expressed in terms of the ohms resistance per cubic centimeter of the substance at 20° C (68° F).

Usually, a material's resistance increases with if the temperature is higher because electron agitation increases also.

Take a look to the following animation:

 fig.3 - Resistance (animation)

Imagine that the white disc is the current flow and the other discs are the electrons inside the material.
If electrons increase speed then the collision force increases also determining the amount of resistance that the material causes.