Opt-In: Opt-in is when a subscriber or customer requests to be added to a company's email list, giving the company legal permission to send them these informational emails.

Opt-in email lists are used by most, if not all, direct marketing firms and Internet marketers; typically, these marketers use applications that allow them to manage subscriber lists. The company or business will continue sending email communications to the customer until he or she chooses to opt out of them.

Email marketing has been proven to generate sales and profits effectively by several studies. Customers prefer it -- about 77% of consumers reported preferring permissions-based marketing emails to any other form of advertising. In addition, according to one survey, approximately 44% of customers subscribed to an opt-in email list made at least one purchase in the last year because of these emails.

And as the benefits of email marketing continue to reveal themselves, it's likely that more companies will look to offer opt-in subscriptions to their customers. Each year, spending on email marketing and opt-in subscriptions increases by about 10%. In 2013, 56% of businesses in the United States planned to grow their use of these permissions-based marketing emails; it's safe to say this percentage will only continue to grow.