I've heard of this but am not familiar with it.
What exactly is RPG?
A really basic definition would be playing pretend according to rules. Most DS rpgs are run by facilitators. They're a lot of fun, but I haven't participated in any in ages so I hope I explained it well enough.
What would constitute someone doing it?
A lot of us have usernames, avatars, signatures and personal messages taken from DS, and we encourage that. But if in your messages you're pretending to be that character, as with Sarah looking for her relatives or her doll, you're role playing. RP is fictional storytelling, same as fan fiction, except the author puts him/herself into their writing, and like FF there are websites dedicated to it.
From the User Agreement/Guidelines:
2. The purpose of these forums is to discuss DARK SHADOWS, its characters, actors and storylines. Role playing (putting oneself or another poster into the DS story) is discouraged, and repeated efforts to role play may result in removal of these posts from the board. This does not, however, refer to DS-related usernames or signatures. These forums also do not accept fan fiction and such works are subject to removal.