Location Mailing Lists are those that pertain to a specific locale. Some of the lists are state-wide, some county-wide. There are even mailing lists for genealogical information relative to a specific city. If your research is place-specific, either in a U.S. state or a foreign country, these are the lists to join.
As far as posting guidelines, each list has its own rules, but generally common sense prevails. You are requested to post messages which are on-topic. For example, if you join a mailing list whose topic is Shelby County, Indiana, post messages about your ancestors and research in Shelby County, not New Orleans.
Next, be polite. We genealogists tend to be a polite bunch, but the occasional odd duck does appear. If someone posts an offensive message, just contact the list administrator and let them deal with any problem. Carry on personal correspondence with other group members off-list, via e-mail, if what you have to say is only of interest to you and the other person.
When you subscribe to a mailing list you will automatically receive an e-mail detailing any rules regarding posting to the list, plus information on how to unsubscribe. Make sure you keep this initial e-mail, otherwise if you want to unsubscribe, you won’t remember how.
When you join mailing lists, the chances are very good that someone on the list will be researching one of your surnames, or one of the locations your ancestors lived, i.e. a particular county or country of origin. Mailing lists optimize your chances for success. Many researchers have made the most progress in climbing their family tree thanks to mailing lists.