A list refers to a set of subscribers who have agreed to receive mail from you.
A Campaign refers to a mailsshot you are going to/have made.
Mailchimp handles all aspects of doing this with ease.
It has a number of sophisticated features which allow you to:
- design newletters
- segment/filter your lists
- create a registration form with additional fields
- auto generate mails for new subscribers (paid for)
Mailchimp is free if you have fewer than 2000 subscribers and send less than 12,000 emails per month. In addition to providing a professional legal mailshot service, it also uses its own server for sending email so reducing the pressure on your own ISP/web service.