Couldn't have said it any better.Impedance is a complex quantity. It has a REAL part and an IMAGINARY part. The REAL part is called resistance and is not frequency dependent. The IMAGINARY part is called reactance and it is frequency dependent. The MAGNITUDE of the impedance is the square root of the sum of the squares of the REAL and IMAGINARY parts. The phase angle of the impedance is the arctangent of the IMAGINARYpart over the REAL part.
http://en.wikipedia.org/wiki/Complex_number
by Jake Hertz
by Steve Arar
by Gary Elinoff