What is the difference between a Jus, Sauce and a Gravy?
What’s the difference between gravy, jus, sauce and coulis?
  
  Will says...
  
  •    'Gravy is made from the juices of oven roast meat, thickend with
  a starch such as corn flour.  
  
  •    Jus is made from the same juices
  that has been refined and condensed to get a clear liquid naturally
  thickened.  
  
  •    Sauce could be made from the same juices with other
  ingredients added such as wines or spirits to compliment the food.
  Sauces can be served hot or cold.
  
  •    Coulis is a thin fruit or
  vegetable purée, used as a sauce. If the purée is too thick, you can
  add some water and sometimes sugar'.


Source: http://www.41portlandplace.com/news/116/send-head-chef-will-carvalho-your-questions-about-the-kitchen

My personal feeling for sauces is they can be made from anything. Parsnips, Mushrooms, Celeriac etc when 'Gravy' and 'Jus' must have Meat juices as their base.

On menu's you'll often see 'Gravy' on pub menu's where they aren't trying to sound to posh, probably using some sort of packet mix. Jus' on French style menu's OR people trying too hard. Finally you'll see something like "Beef and Juniper Sauce" on my menu and many others because... Gravy sounds too common and Jus sounds too poncy.