Relationship between real and imaginary part of a real-valued and causal system

1. What does the real and imaginary parts of the FFT signify?
2. What does the imaginary part of a Fourier transform mean?
3. What is the difference between causal non causal and anti causal system?

What does the real and imaginary parts of the FFT signify?

The real portion of an FFT result is how much each frequency component resembles a cosine wave, the imaginary component, how much each component resembles a sine wave.

What does the imaginary part of a Fourier transform mean?

If you consider the input as current, the transfer function or Fourier transform as impedance then the output is potential. If Fourier transform is impedance, then the real part of FT is resistive part of the impedance and imaginary part is the reactive part of the impedance.

What is the difference between causal non causal and anti causal system?

A causal system is one whose output depends only on the present and the past inputs. A noncausal system's output depends on the future inputs. In a sense, a noncausal system is just the opposite of one that has memory. How can a real-world system be noncausal?