Bad Eggs: The egg on the left shows a ring at 6 days. This ring is formed by concentrated bacteria which has invaded the eggs' membrane. It can become present very early, or after the chick has already started to form, as in the picture on the right. In the picture on the right the ring, or portion of it, can be seen at the bottom of the egg with the expired chick in the middle.

More Bad Eggs: For a different reason. The egg on the left shows a blood spot. In my experience an egg with a blood spot will not hatch. They will go bad and blow up, though. The egg on the right at 6 days shows "clear". It is infertile, or too old to germinate.

Yet Even More Bad Eggs: The egg on the left shows a blood spot incubated to 8 days. You can see the bacterial ring forming at this point. Soon this egg will start to "weep", and if it isn't caught in time, it will explode into a stinky mess. The egg on the right shows highly defined pores. Eggs that look like this under candling have a slim chance of hatching in my experience. I've noticed that it mostly depends on the severity of the porosity.

Good Eggs: On the left, you can see the "spider" of veins growing away from the peep. This egg is at 6 days. You can see this spider in a smaller version at 3 days. The egg on the right is at 2 weeks. You can see the clear spot beneath, with the yolk and peep floating at the top.