 The Fourier series tells you the frequency response of repeating signals. The Fourier transform tells you the frequency response of non-repeating signals. Both the Fourier series and the Fourier transform test signals with sinusoids at many different frequencies to tell you which frequencies your signal is built out of. A sinusoid is a repeating waveform. Adding together sinusoids that are close together in frequency causes them to interfere with each other. By controlling the amplitudes of sinusoids at adjacent frequencies, you can cause the waveform to cancel out at certain points in time. The closer together the sinusoids are in frequency, the longer it will take for your waveform to repeat. The Fourier transform models non-repeating signals by testing sinusoids that are infinitely close together in frequency, ensuring that the waveform never repeats.