They are a very powerful marketing tool. Here's how they work, you find a list you want to join, sign up, and however often it goes out you will receive an email message in your box with everyone's comments. They are very good for getting your name out there in Cyberspace and increasing your visibility online. The idea is to be helpful to others on the list by answering questions or giving advice, your sig line will do the rest. You can also make a lot of good contacts and find sales leads by monitoring a lot of lists.
Lists come in two forms moderated and unmoderated.
Moderated lists are usually better since all posts are looked at by the moderator before going out to the group. This helps cut down on spam being posted. Mailing lists are similar to a discussion on a news group, but differ in that you don't have to go to the board to follow the discussion, it comes to you through email. They really are a wonderful tool for networking on the Net. The email List Owner Resource Network, very extensive.