Contrast is a very subjective thing... Basically, you are talking about the ability to tell tone from tone, the more contrast, the less middle tones you see, the less contrast, the less extremes (pure white or pure black) you see. Good or bad are imprecise words in and of themselves, so an accurate description of "good" contrast is hard to come by. In practice, good contrast varies with each person's perception of what a print should look like.

The following picture is quite low on contrast, in part because it was a cloudy day, in part because it was underexposed, in part because it was developed in Diafine. Diafine is a two bath developer, so highlights keep more detail than they would otherwise.

Although it has lower than normal contrast, it still communicates very well, so contrast is not necessarily bad. The lower contrast simply alters the mood.

This next shot has good contrast, to my eyes at least. It was also developed in Diafine, but in a better light, and correctly exposed. To me, the contrast is good because all the tones are right where they should be for an accurate rendition of the scene. For some reason the previous sentence smells of BS to me. Maybe accurate rendition is not the phrase to use, but you catch my drift.

The attached picture would be the high contrast one. I printed it at about a grade one paper, and the highlights still lost detail. To me, this is the classic "bullet proof" negative. Besides being shot in full (noon) sun in N.E. Brazil, it was over developed (HC-110). This print is still acceptable to me, but the loss of highlight detail is noticeable.

In all of these, however, the mid-tones where my area of concern when printing. It is my belief that if those are good, minor problems in the extremes can be more easily forgiven.

As to manipulating contrast, it can be done by manipulating shadow detail in exposure, or highlights in development. Increase exposure, and you get more shadow detail (shadows slowly become midtones). Increase development and highlights become brighter (denser negatives). This is very well explained by Ansel Adams in The Negative. Regardless of your taste for his photography or not, the book is worth a read. I personally don't care for most of his work, but own a copy of The Negative.