List building is a method to collect targeted prospects who are visiting your site by offering them a decent quality information in exchange for their contact details such as email, first name, last name and their phone numbers, this is also known as laser targeted, responsive subscribers list that eventually becoming the key element to your success.
When the subscriber has confirmed his email subscription on your list, you can then start to send them the follow up emails in conjunction with relevant and useful content based on your product or services.