I am certain that someone here can answer this in more detail than I. I'll try and start things off though on constructing an answer.
My understanding is that with the German conscription and service system, troops were recruited, trained and served for a set period. I believe specialist arms such as cavalry, artillery and the navy had a term of enlistment a year longer than usual, so three instead of two if I remember correctly, likely having to do with them needing more time to become proficient and then the Kaiser still needing X amount of service from them.
After being released back to civvy street, they were assigned to a Reserve unit for X number of years. This unit would conduct periodic training and was called up on mobilization exercises and for field manouevres. German Reserve units thus retained a higher degree of proficiency than Allied Reserve or Territorial units. Again, if I remember correctly, reservists at a certain age ended up automatically transferred to the Landwehr.
Reserve Divisions were identical in composition to Regular Divisions save for lacking one (or two) battalions of divisional artillery. Presumably Landwehr and Ersatz units had different scales of issue and thus tables of organization to Reserve units, possibly fielding more antiquated equipment, but undoubtedly the Kaiser was not doling out Dreyse Needle-Rifles to the Ersatz or Landwehr battalions by 1914.
Because the differences in Regular and Reserve were so minimal, commanders were able to use the formations interchangeably. This gave the Germans what I feel is a decided operational advantage, but the development of operational art and tactical methodology during this time was not such that it could have been capitalized on. Landwehr and Ersatz units may have been radically different, but if their employment seems to be interchangeable as with Regular and Reserve units I would surmise that they are quite similar in TO&E, though probably lacking in artillery and possibly, by the Ersatz or Landwehr stage, machineguns also.