Impedance is the total counteractive force to the flow of electrons through a system. You can think it to be the resistance of the circuit and it is measure by the use of an impedance analyzer. There are no resistors, components, inductors or capacitors that are purely reactive or resistive. There are some traces of R, C, and L aspects in all these things and these unwanted aspects are called parasitics. When there are specific frequency requirements in high speed applications these parasitics need to be accounted for.
The impedance analyzer will provide a variety of information about the load under test from the impedance, inductance, admittance, phase, capacitance and specifications and matching impedances.