Impedance (Z) is the property of a circuit or cicuit element that opposes current.
There are two types of Impedance:
1) Resistance (R)
Resistance is impedance arising from the conversion of electrical energy to heat or radiation and consumes energy.
2) Reactance (X)
Reactance is impedance arising from the storage of electrical energy in an electric or magnetic field and does not consume energy.
Therefore
Z = R + X
Impedance, and both of its components are measured in Ohms (Ω) which sometimes leads to confusion. The most common misconception being that knowing a circuits resistance is all that's required to calculate how much current will flow. This is compounded by the generic use or R where Z is meant. To understand why this is so it is important to realise that Reactance is typically in the order of 100 times smaller than Resistance, in a perfect circuit Impedance = Resistance and there is zero Reactance.
Ohm's Law 

One Ohm equals an opposition of one Volt per Amp (Ohms Law) R = V / I 

U or V = Potential Difference in Volts I = Current in Amps R = Impedance in Ohms (often misquoted as resistance) P = Power in Watts 
Any Voltage Source (ac or dc) has an impedance associated with its ability to deliver a maximum current. This value is called the "Source Impedance" and can be represented as an equivalent circuit as below:
Thévenin Equivalent Circuit 

A perfect Voltage Source V_{Th} is one whose output does not drop under load. In practise any source has an internal or Thevenin Resistance R_{Th} called the Source Impedance. The perfect voltage source is equivalent to the no load or open circuit Voltage. To calculate Source Impedance connect a known resistor to the source and measure the current. if load resistance is known R_{Th}=(V_{Th}/I)R_{L}If output voltage is known R_{Th}=(V_{Th}V_{out})/I 
Source Impedance is crucial in calculating Fault currents in any circuit and is given the symbol Z_{s} in the IEE wiring regulations BS7671 (currently 16th edition renewed 2001)