Tim, sorry for the frivolous asides. :-[ :-[
Bouillon, like stock, is the liquid produced by boiling meat and/or vegetables in water. They are synonyms for practical purposes. Bouillon is not gravy but like stock gets used in the process of making gravies and sauces. Not to be confused with court-bouillon which is the liquid used for cooking fish. It is not soup except in the sense that the old-fashioned "beef tea" is soup.
Like others in this thread I prefer to make my own, however, I don't always have the time to make it so resort to buying powder, cubes or ready made liquid stock. My criteria for choosing are:
do I need beef, chicken or vegetable
which contains the least salt and/or additives
where possible I buy organic
I always read the packet before buying. (Come to think of it I'd probably be quicker making my own!! ;) ;D)
As far as I am concerned the use of the word "bouillon" by manufacturers to describe stock is an affectation - a marketing ploy to attract people who might think that they are buying a "classier" product when, in fact, they are simply purchasing stock.
Hope this helps! :)