Overview
The theoretical treatment of devices such as capacitors and resistors tends to assume they are ideal or "perfect" devices, contributing only capacitance or resistance to the circuit. However, all physical devices are connected to a circuit through conductive leads and paths, which contain inherent, usually unwanted, inductance. This means that physical components contain some inductance in addition to their other properties.
An easy way to deal with these inherent inductances in circuit analysis is by using a lumped element model to express each physical component as a combination of an ideal component and a small inductor in series, the inductor having a value equal to the inductance present in the non-ideal, physical device.
Read more about this topic: Equivalent Series Inductance