If you have a plot of a voltage signal as a function of time (how the voltage changes with time), the function is in time domain. You need to use Fourier transformation to get this into the frequency domain.
When you do the Fourier transformation, you will get a complex result. Although the original voltage function was real, the Fourier transform is complex. Mostly we use the modulus of the complex quantity.
The x-axis after the Fourier transformation becomes frequency scale (it is now proportional to 1/t) and is often called the power spectrum. Another Fourier transformation will get back the original voltage versus time graph.
Not quite so; if you use the modulus of the complex quantity, you will now get the voltage that is complex now. If you convert the complex number to the polar form (amplitude and phase), you will get another result. All are related but are often difficult to figure out just by looking at it.
Transformation of log quantities are another matter altogether.