One is a newsletter. This works like any newsletter you would get from a school or church. Most web sites have these. The second is just a discution group thru your e-mail.
Any body can write an e-mail and it gets sent to everyone on the list.
In both cases the e-mails your get are an a specific topic or are updates about a site. Make sure you stay on topic when you write to a discussion group. There are two great things about e-zines. E-zines came in two forms. One is a newsletter.