What are Newsletters and Ezines?

In some sites you can find the expressions: "Subscribe our Newsletter", or "Get our Ezines" and I wondered what exactly were this. I found that they are another form of writing, included in e-mail marketing, that has been developed by certain publishers to reach their niche. This frequent communications about news, products or services are called Newsletters and Ezines.
Christopher Knight, in the article "What Is An Ezine? E-Zine? Email Newsletter? E-Newsletter?" explains that an Ezine is an e-mail newsletter or e-newsletter, not Web-based only, sent by a publisher that has your permission. Enzines is a newsletter that is delivered via email that you can subscribe or opt-in to it. The same author states that Ezines are useful tools to develop relationships within your niche, to keep in contact with your clients or members, build your credibility or perceived expertise in the marketplace and drive leads, sales and/or revenue by selling products/services or advertising along-side quality original content.
In the Internet you can find many Enzine Directories. I found Christopher Knight, an article writer, in a site about e-mail marketing, called E-mail Universe. This site has good information and resources about Ezines.