You should realize, that the completely determined wave, such as s(t) = Acos(wt+phi) doesn't contain any useful information, because all the parameters of the function given are determined and exact, therefore there is no need of transmitting such signal, as no new information will be obtained from it at the receiving side (everything, including amplitude, phase and frequency, is known).
However, if one of these parameters changes somehow, we say, that it's modulated in accordance with the informative message. Thus, different types of modulation become actual and they correspond to time-varying of different parameters. At the receiving side we demodulate the signal and find out the law of information changing. This changing may be connected with different physical processes, such as, for example, temperature, humidity, pressure varyings and etc. The key idea is that WE GET KNEW KNOWLEDGE.
With respect,
Dmitry