DMChannel is a private conversation between the bot and a user. It is what channel.isDM() gives, the channel of a private message and of a command used in a private message, and what user.createDM() returns. To write to someone, user.send() is shorter.
How to send, edit and read messages is in Sending messages. Private messages only reach your bot with
chapterjs start: see Private messages.Properties
It also has everything of Channel and what channels with messages share.guildId and guild are null: there is no server.