How does Thunderbird "decide" what is junk?

When you receive emails, Thunderbird has a "junk classification" process that is used to determine what is junk and what isn't. You can "teach" it by identifying emails yourself, and Thunderbird will then use these suggestions in future junk sweeps.

Now there are 2 stages to this:

1. A little flame icon will appear next to an email, suggesting that it might be junk, yet it will stay in your inbox until you identify it as junk.
2. It will review your emails and identify what is junk; and move it to the junk folder automatically.

So what factors are considered when Thunderbird decides what is/what isn't/what might be junk?