There's been some previous discussions on this. Take a search through the B&W forum here and you should find them.
Changes in contrast are very important in B&W photography. If you look through the gallery sections, you'll plenty of examples, good, bad, and average. Still its hard to describe. Maybe someone more eloquent can explain. I will simply post two examples.
The first example is what I would call good normal contrast. Plenty of range between the highs and lows. This particular one has only a few areas in between.
The second example is very low contrast. So low that I had to pre-expose the paper. The low contrast was caused by a huge burst of steam just before I pressed the shutter. I had set the exposure before the steam burst.