Equivalent Series Resistance

In an ideal world capacitors would only exhibit capacitance and inductors would only exhibit inductance. However, in reality both capacitors and resistors are not perfect and exhibit other qualities, including resistance. To account for this in circuits it is useful to consider capacitors and inductors as having a resistor in series with them. This resistance is called Equivalent Series Resistance (ESR).

There are different reason as to why these components have ESR but one is that any material that is used to create them will have a resistance. Also, theoretical capacitors and inductors do not dissipate energy. However, real world ones do and therefore contain resistance.