What is a Distribution List?
A distribution list is a group of mail recipients that is addressed as a single recipient. Distribution lists are used to send email to groups of people without having to enter each recipient's individual address.
Where can I find them and what do they look like?
Address Book
You will notice that a distribution list has a group of people 
beside the name, where a single email address has one person
beside it.
Email to Line
In the email, you will see a 
this lets you know this is a grouping of email addresses.