Listserv is a brand of Email software that works as an automated distribution list.
We actually use a product called Mailman but Listserv is often used when referencing any software of this type. Kind of like Kleenex. Anyway, whenever a message is sent to the list's email address, the message is then forwarded to all the subscribers of the list. The subject line is modified to include the name of the list (This is not true of all lists but ours are configured this way). This should be the first clue that the message is coming from the list and therefore going to all the list subscribers.
When you reply to a message that came from a list it goes back to the list and all the list subscribers by default, unless you purposely change the "to:" field in your email software (Eudora, Outlook, etc). Listserv lists are usually geared towards a specific audience. There are established lists on virtually every topic imaginable. Most messages sent to a list are written to address everyone in the group.
As a conversation begins and continues (we call this a thread), it may transform into a dialog between individuals but the writers may continue to send the messages through the list because they know that others may still be interested, even though they are not actively participating in the conversation.
At other times it may be more appropriate to continue a conversation outside of the list. There is no wrong way or right way to use a list, although if you send something to a list that was intended for an individual it could potentially be quite embarrassing. Listserv lists are a valuable communication tool. As with any other technology, it sometimes takes people awhile to figure out the proper protocol and terminology.
Thank you for your time. End of rant. };-)>