What exactly is the difference or relationship between power spectral density and power density function? I thought they referred to the same thing but one article I need says to calculate both of them so now I don't know if they are different. A layman's explanation or explanation in terms of physical meaning would be better. Also what is it's relation ship with periodogram? If X(f) is the DFT of x(t), then is $|X(f)|^2$ same as periodogram?