The perfect example of this is the signal received by an AM radio:
Notice that there are two waves which obviously have different frequencies that are superimposed to form the AM modulated signal on the bottom. It can be proven, with a lot of calculus, that any periodic function can be written as a sum of sines and cosines, each with a different frequency. This is the basic idea of something known as Fourier analysis. Here is a more complicated example of some sines and cosines added together to produce what looks like a square wave. You can imagine that, given an infinite number of sines and cosines, eventually you'll reproduce the original wave.