Astronomers describe the ISM as turbulent, meaning that the gas has quasi-random motions coherent over a large range of spatial scales. Unlike normal turbulence, in which the fluid motions are highly subsonic, the bulk motions of the ISM are usually larger than the sound speed. Supersonic collisions between gas clouds cause shock waves which compress and heat the gas, increasing the sounds speed so that the flow is locally subsonic; thus supersonic turbulence has been described as 'a box of shocklets', and is inevitably associated with complex density and temperature structure. In the ISM this is further complicated by the magnetic field, which provides wave modes such as Alfvén waves which are often faster than pure sound waves: if turbulent speeds are supersonic but below the Alfvén wave speed, the behaviour is more like subsonic turbulence.