There's a variety of ways to describe form with a pencil, and the most common is 'contour'. This is a linear approach to drawing, focusing on creating descriptive outlines. The trick is to draw the unique contour you're observing, and not a generalized outline drawn from your memory. Beginners often fall into the trap of drawing 'symbols' instead of actual objects -- even if they are sitting down with an object directly in front of them. As children we're taught to draw symbols, and becoming an artist requires you critically look and record.