Power spectral density plots shows you the power (energy) distribution with respect to frequency of the input signal. For example a pulse-like ground motion you'll have most of the energy concentrated at the predominant pulse period. The complete opposite of the pulse-like motion is the white noise - it has a constant power-spectral density over all frequencies.

Specific energy density is the integral of the square of the velocity time history. It is a measure of the overall energy of the record (larger SED, larger energy and expected damage), but it is not used often, Arias Intensity and Cumulative Absolute Velocity (CAV) are preferred.