'Foreshortening' is the term artists use when a three-dimensional object is being represented from a certain perspective in a two-dimensional space and needs to be adjusted or 'shortened' to appear realistic. For example, imagine drawing a picture of a dog that is starting directly at you. Yes, the dog might have a long muzzle, but when he is staring at you, it looks visually shorter and the length isn't prominent. This is when you have to use foreshortening to imply a long snout that might definitely be there, but is not perfectly represented in that angle. This continues onto most other three-dimensional objects in figure and still life drawing, but it is perhaps most common when drawing human limbs.
Here is a brief tutorial with just one technique on how to handle foreshortening in life drawing, using the 'seven-and-a-half' heads proportion strategy I posted in a previous card ("Quick Tip: How To Draw Perfect Proportions").
(Warning: He's not the most engaging speaker, but I feel like he's pretty clear and brief at explaining this technique!)