The document discusses different types of convolution operations on signals. Convolution involves taking one signal, flipping and shifting it, then multiplying it sample-by-sample with another signal and summing the results. Linear convolution produces a output signal that is the sum of the two input signal lengths minus one. Circular convolution treats the signals as if they repeat periodically, producing a periodic output signal of the same period. It can approximate linear convolution if the period is long enough. The discrete Fourier transform (DFT) and its fast version, the FFT, can also be used to efficiently compute circular convolution by multiplying the signals' Fourier transforms.