An electronic 'mailing list' is a collection of people with an interest in a particular subject, where an email sent to the mailing list address is resent to all the people on the list. Many lists are set up to cover a multitude of subjects, and in genealogy, most are based upon surname or region, although a few also exist that cover subjects like periods of history, emigration, etc. Mailing lists are ideal for asking questions about your ancestors and for making contact with unknown cousins that may be pursuing your common ancestors. You may prompt a discussion by posting to the list to see if anybody else is connected to you, you may help others by responding with whatever relevant information you may have, or you may learn by just reading.
Mailing lists are a safe way of talking to other people.
Viruses cannot be transmitted by the email itself, only as part of an attachment to the email. Rootsweb does not allow attachments to be posted to the list, thus protecting the subscribers. The addresses of people subscribed to a list are not available publicly, which means that being a member will not make you more susceptible to spamming. The only time your email address is visible to others is when you post to the list, and even then it is only visible to the other subscribers. To join a mailing list, the person needs to 'subscribe' to the list with a valid email address.