The purpose of this article is to characterize the various kinds and specificities of music representations in technical systems. It shows that an appropriate division derived from existing applications relies in four main types, which are defined as the physical, signal, symbolic and knowledge levels. This fair simple and straightforward division provides a powerful grid for analyzing all kinds of musical applications, up to the ones resulting from the most recent research advances. Moreover, it is particularly adapted to exhibiting most current scientific issues in music technology as problems of conversion between various representation levels. The effectiveness of these concepts is then illustrated through an overview of existing applications functionalities, in particular from examples of recent research performed at IRCAM.