Signals are manipulated by systems. Mathematically, we represent what a system does by the notation y (t)= S (x (t)), with x representing the input signal and y the output signal.
This notation mimics the mathematical symbology of a function: A system's input is analogous to an independent variable and its output the dependent variable. For the mathematically inclined, a system is a functional: a function of a function (signals are functions).
Simple systems can be connected together one system's output becomes another's input to accomplish some overall design. Interconnection topologies can be quite complicated, but usually consist of weaves of three basic interconnection forms.