Usually delivered via email like an ezine, a discussion group, or list differs from an ezine in one major way - they allow you to voice your opinion, usually through a moderator. There are discussion lists on many subjects and they're a great way to get answers to specific questions about your particular area of interest. Discussion lists hold a marketing value similar to that of newsgroups. You help others while your sig file does your marketing. Discussion groups are much more powerful though, since they are delivered to a targeted audience, rather than having to find them.
Many discussion groups offer advertising as well, usually in the form of sponsorships.
This is a moderated, global discussion on internet advertising, marketing and online commerce. If you're interested in online marketing, I highly recommend you check out this list. An excellent list indexed by subject. The information in this tip should keep you busy for a few hours or more. What are you waiting for.