What is a List Rule and when would I use one?

A List Rule allows you to construct a new segment using the standard and custom fields found within your existing mailing lists. You can select one list or multiple lists to define your search criteria and then find matching results.
 
You'll want to use a list rule if you trying to find records that have a specific field value. For example, to find people who bought a specific product and who live in California, New York and Texas you could construct a segment that focuses on a custom field called Product and the standard field called State.