The most important form of communication between player characters is in the form of messages. These represent letters, personal conversations and all other exchanges of words.
Like real-life conversations, messages can go to one or more readers/listeners/etc.
Every message is part of a conversation. These are basically convenient ways of grouping messages, like topics on a forum or threads in e-mail. Conversations have a topic or subject to help you sort things, and participants are set on the conversation, not for each individual message. You can also invite more participants into an existing conversation.
All the Realms also have a special realm-wide conversation where all members of the realm are automatically added (and removed, if they leave the realm). This is useful to reach everyone in the realm with a message.
Conversations can also branch into sub-topics, which again is mostly for convenience.
When replying to a specific message, the game will show replies sorted under the message replied to so you can easily see what the sender is referring to. This makes it unnecessary in most cases to include quotes, which adds just so slightly to the atmosphere as people didn't generally quote in handwritten letters.
TODO: Include formatting guide.