A customer list is simply a group of customers that share some common features, such as a list of all California residents.
A list is a basic unit for operations, such as performing a mass update or mass delete and to run a campaign.
When you import a list, all the customer records from the import file will share the same Lead Source. The default is usually the import file name.
When you search customers based on some criteria, such as a certain zip code, the result is a list of customers. You may save the search and use it in your campaign. Actually, an import list is just a special type of search list (all have the same lead source).
Campaign result can be saved as another list. For example, you can save all customers who expressed interest into a new list for another campaign.
The following video shows how to use advanced search to create a list and then start a campaign.