The Timestep class is used to hold the dynamic data for a Molecule for a single step in a trajectory. It hold the coordinates, velocities, energies, etc. that vary with time. As each new Timestep is read in from some source (a file, or from a network connection) it is added to the animation list for the associated molecule; this list is maintained by the Animation object, from which each Molecule is derived.
Almost all the items in this class are public, so they may be retrieved quickly by other objects in VMD. This is not such a great design, however; see the notes below about future changes.