An ezine is an email newsletter that's sent to people who have subscribed via your opt-in form to receive it.
It can be a very successful tool for communicating with your customers, building a relationship with your visitors, branding yourself and your site, increasing your traffic, launching your own products, profiting from affiliate products, and more.
You probably subscribe to at least a few ezines from other publishers.
You've probably also quickly unsubscribed from several ezines.
What makes you stick with some ezines but drop others like a hot potato?
The ezines we stay with tend to be ones that provide useful new content and resources. We drop the ones that are nothing but sales pitches or canned / regurgitated content.
And that's the key to a successful ezine: Provide original, useful content - and preferrably, provide a lot of it for free. Always give (in the form of free, useful content) before you take (in the form of recommending a useful product or service the reader has to pay for).
Make sure some of the messages you send to your list contain no monetization at all - simply drop a line to share some valuable information for free. That's what makes you a trusted, indispensible resource for people in your niche.
Based on your research and your ongoing efforts to tune in to the conversation of your niche - what original, valuable content would your subscribers love to receive? What format would they prefer?