The client will send this packet when the “enter” key is pressed after typing a chat message. The vanilla server will parse this chat message to see if it starts with the character "/", at which point it will assume the chat message to be a command. Otherwise, it will send it to the other players in the server. The server can also colorize chat messages by prepending the section sign and the color code to the message (e.g. §cR§6a§ei§an§bb§9o§dw). See Plancke’s website for all the color codes. Text styles cannot be sent.
Warning: A client sending the vanilla server a chat message longer than 119 characters might cause the client to be kicked.