Filtering is just recasting these ideas into the frequency domain. A coffee filter allows small particles of coffee to pass through, but keeps back the larger coffee grounds. In signal terms, the components are sinusoids rather than particles, and the selection of which to allow through is based on their frequency rather than their size. We can make a four-way division among signals. Firstly there is the distinction between Periodic and Aperiodic signals.
Periodic signals have a waveform shape that repeats in time.
That is it is possible to predict the future of the waveform by looking at its past. One can isolate a region of the signal, called its period, which occurs over and over again in time. These sounds also give us a clear sensation of pitch. Aperiodic signals have waveform shapes that do not repeat, that is it is impossible to predict the future of the waveform by looking at its past. Simple periodic signals are just sinusoids, while complex periodc waveforms are combinations of sinusoids.