In algebra, what is the difference between rule and roster?

In algebra, the definition of rule is a description identifying the members of a set. The definition of a roster is how you specify a set when listing all members of a set. I kind of get the gist of this, but do you think you could explain it in greater detail and/or give examples? I'm kind of confused on how to identify this (aka, if I take a quiz over it sometime. Which may be in the near future. Like tomorrow.) Help is appreciated!