I'm not really an expert, but in basic terms:

Roast darkness: 

 - Roughly how long was the coffee roasted, much like toast, a longer roast usually brings about a darker flavor. Darker roast coffees tend to have a slightly burnt caramelized flavor.

Body:

 - I usually think of this as more of a texture than a flavor, probably more easily noticed at the extremes. A "full bodied" coffee sort of coats the tongue causing the flavor to linger while "medium bodied" coffees tend to be a little lighter with a cleaner finish.

Acidity:

 - Acidity is basically what it sounds like, lower acid coffees are often referred to as "smooth" and higher acid coffees referred to as "bright".

Now [as mentioned in the comments][1] these terms are really painfully relative, and somewhat rendered meaningless by an onslaught of advertisers claiming that their coffee is "smooth dark and full-bodied" regardless of the real characteristics. 


  [1]: https://coffee.stackexchange.com/questions/52/roast-vs-body-vs-acidity#comment56_52