An opt-in is a form on your website that allows people to give you their email address. You might also collect additional information, like their name, but the email address is the essential component.
Opt-in is also a verb that refers to people taking the action to give you their email address and giving you permission to email them. “I opted-in for your email newsletter”.
Companies use opt-ins in order to build a highly targeted email newsletter lists they can later market their products and services to.
Opt-ins and Newsletter Signups serve the same function: to collect people’s email addresses. However, what you give people after they sign up depends on your strategy. Newsletter signups imply that you will be sending out a regular newsletter to your email subscribers.