For Dog House chat, see this link
Chat is multichannel; Global (worldwide), Local (100 block radius)
To chat in a channel use:
/ch g (for Global)
/ch l (for Local)
/ch list (List all channels)
You can also send chat to the other channel without leaving your current one. For example if you are chatting in Local, you can do /ch qm g <message> to send a one-off message to Global (and vice versa)
/msg player message - Sends a PM to the named player
/r message or /reply message - Replies to the last PM you received
You can use colours in general chat and PMs using colour codes. For example: &cThis will type in red
Type /ch help in-game to see a list of commands. You can get more detailed information on a particular command by providing a question mark (?) as the first and only argument, i.e. /ch join ? will provide more information on the join command.
/ch g - Join Global and start talking there
/ch qm gst - Send a single chat to the Guest channel without leaving your current channel
/ch who - See who else is listening in your current channel
/leave gst - Stop listening to the Guest channel
See the Chat Channels article for a list of channels, their aliases and descriptions.