= Solution
A conventional <capacitor> stores separated electronic charge across a dielectric, with capacitance set approximately by $C=\varepsilon A/d$. An <electric double-layer capacitor> stores charge at the interface between a porous electronic conductor and an ionic <electrolyte>. Electrolyte counterions screen charge on the electrode over a very small interfacial separation, and a porous electrode provides a large accessible area. The separator prevents an electronic short while permitting ionic transport; the two electrode interfaces form series capacitances.
\b[<Pseudocapacitance> is charge storage by fast, reversible Faradaic processes that give a capacitor-like relation between stored charge and potential over a useful range.] Examples include surface redox and suitably rapid insertion processes. It is distinct from purely electrostatic double-layer charging. Merely observing a redox peak does not establish pseudocapacitive behaviour: a slow two-phase battery reaction can remain battery-like even when its electrode is nanostructured.
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