There really aren't many perfectly straight lines in nature - as they are mostly of human invention, found in machinery, architecture, etc. However, straight lines help to convey rigidity and firmness of form, whether something is absolutely solid or not. Straight lines also denote tension, such as the tautness of a rope pulled tightly, flesh stretched tight over the bone, or the creases in a freshly pressed suit and pants.
The 'C' curve travels in one direction and is mostly employed to show fullness of form. It can portray soft, pudgy flesh and the puffiness of fur. Anything that is inflated with air or bloated with liquid tends to round out into 'C' curves. The effect you can create when using them on humans or animals can also result in personality types that are friendly or comical. (Think of all of the rounded puffy forms on a circus clown, for example.)
The 'S' Curve is a curve that starts out in one direction then changes and curves in the other direction. This type of line is very prevalent in nature and is used to show rhythmic gracefulness of form. Animals that we consider very elegant in their structure, like cats and many types of birds, have flowing forms full of 'S' Curves. And of course an attractive female figure is loaded with them too! Many things in nature also move in 'S' Curve patterns, such as seaweed undulating with the current, a figure skater or ballet dancer, or a squirrel bounding up and down through the grass. A snake has to travel in 'S' Curves, its body pushing off from side to side through complex muscular contractions in order to propel itself forward.
From the examples above, I hope you can get a sense of what type of line will best suggest the desired form. Though I have deliberately used a preponderance of each individual type of line in the respective examples to exaggerate my point, a good drawing should ideally comprise a variety of linework stressing all three types of line. This not only helps to convey the correct form, but also creates visual variety, which is more pleasing to the viewer's eye, ultimately helping to engage their interest. This still of Shere Khan the tiger, from Disney's "The Jungle Book", has a nice variety of straight lines and curves that suggest exactly what the form is.
Here's an example from TV animation that still illustrates the principle of well-chosen lines to convey form. There are a few straight lines on Fred and Barney to show more of their ruggedness relative to their softer wives and round, chubby babies.Unfortunately, many of today's cartoon shows fail to recognize the importance of line and tend to design everything with too many straight lines and sharp corners. Yes, I share John K's extreme distaste for this supposedly "stylistic" approach, as there really isn't anything clever or appealing about it in my view. Here's an example of what I mean:
This character has been created almost entirely from straight lines. Even his tongue! 
The result is that the character looks like he's been chiseled out of stone rather than made up of flesh, muscle and hair. The image ultimately has no sense of weight or volume and is merely a flat graphic design, and not a very good one either, considering the directionless arrangement of the lines in the hair for example. It certainly has not been designed for anything more than stiff, mechanical movement either, and this self limitation makes me long for the days when characters were designed specifically to work well in flowing, organic animation. Visually, it has all of the wit and appeal of a connect-the-dots puzzle in a kids' activity book! Sadly, we're seeing more and more witless design in today's TV animation. The tragedy is that it doesn't have to be this way, as there are countless individuals toiling away within the industry (and outside of it too) who are capable of far better design themselves. Why are they not being given a chance to shine? Why this rampant trend toward mediocrity?
