Animation is a base class for all Molecule objects in VMD. It is responsible for storing the dynamic data for molecule - those items which vary with time, instead of remaining constant. Animation stores an animation list, and has controls to add new data to the end of the list, as well as to position a current frame pointer in the list. Each time a Molecule is prepared (by calling it's virtual prepare routine since Molecule is derived from Displayable), the current position in the animation list is updated.
The Animation consists of basically an array of Timestep objects. Each Timestep contains the data for the molecule for a single step in the trajectory of the system.