Ephedrine and pseudoephedrine are both beta adrenoceptor agonists, which means that they partially mimic the effects of having adrenaline.
The difference is that Ephedrine will do much more to dilate your bronchial passages and raise your body temperature (including metabolic rate) than psuedoephedrine will. Pseudoephedrine is more effective as a decongestant.
ECA stacks also include 'Caffeine', which is a phosphodiesterase inhibitor (phosphodiesterase is an enzyme that breaks down cyclic adenosine monophosphate) this is a messenger chemical that is generated within the cell in response to the stimulation of a suitable receptor (such as a beta receptor excited by the presence of ephedrine).
So, what caffeine does is to make the other stimulatory chemicals more effective-ephedrine switches the cellular processes on, caffeine makes them stay on longer.
Knowing that, Caffeine will also cause pseudoephedrine, to be more effective in keeping your nose clear than pseudoephedrine would be alone. Current research does not indicate that this is a major advantage in weight loss, however. So, you're much better off with ephedrine for weight loss.