What are "chat channels"?
Unless you're a totally lone wolf type, talk will be central to your Midgaard experience. Talk, though, comes in different flavors. Here's the lowdown:
Ask. Use the Ask channel to ask questions of other players, administrators, deities, and in some limited cases, NPCs. Ask is independent of RL laws of physics: when you Ask someone a question, it doesn't matter where they are, how much distance separates you, what the intervening geography is, what the weather conditions are, or anything else physical and mundane. Ask and, if they're listening, they'll hear. You can Ask across World boundaries. Note that only the named person will hear your question. Of course there's nothing technical stopping you from asking questions on the other channels, but it's considered impolite to do so.
Auction. Players can auction Items whenever they like. The Auction channel is the medium through which you listen to or participate in ongoing auctions.
Chat. Messages posted via the Chat channel are immediately seen by everyone listening, kind of like an ESP party line. Chats do not cross World boundaries. If you Chat in Midgaard, your message will be received by everyone currently in Midgaard, but not those in TriadCity or other future Worlds.
Congrat. Like Chat and Shout, messages posted to the Congrat channel are immediately seen by all listening players. Use this channel to help someone celebrate an accomplishment, such as gaining a level. Don't use Chat or etc. for this purpose: the Congrat channel is provided specifically so that busy (or grumpy) players can tune them out if they don't want to see them. Like Chat and Shout, Congrats do not cross World boundaries.
Level. When a character gains a level, Midgaard will automatically generate a congratulation notice sent to all players who want to receive it. You can't send a message via this channel, you can only receive them. Use the Congrat channel to help the leveler celebrate their accomplishment.
Page. When you Page someone, you cause a notice to appear telling them you want their attention. In our GUI client this will be a popup message; with MUD clients the message will be written to the ordinary output. Page is useful for contacting administrators, who may be busy doing something else. Like Ask and Tell, Page does cross World boundaries.
Say. When you Say something, you speak it out loud. To hear you, listeners must be in the same Room with you. So, you can use the Say channel to build something similar to "private chats". Just get together in the same Room with all those interested in some topic and have at it. Naturally, Say does not cross World boundaries.
Shout is mystical like Chat and Auction: when you Shout you're heard by everyone listening to the Shout channel, regardless of location in the game world. It's like Chat, but LOUD: you should only Shout when it's appropriate. (When you're being eaten by vampires, for instance.) Like Chat and Auction, Shouts do not cross World boundaries.
Tell means send a private message to a named listener; e.g., "tell Poobah you rock dude!" This sends the message "you rock dude!" to Poobah and nobody else. Of course, Poobah must be listening to receive the message; and must be interested in you to reply to it; neither of which are likely in Poobah's particular case. Like Ask, Tell does cross World boundaries.
Whisper means send a private message to a named listener in the current Room with you, "whisper Poobah Loan me a fiver?" This sends the message "Loan me a fiver?" to Poobah and nobody else. Unlike Tell, Whisper requires Poobah to be in the Room with you. Naturally, like Say Whisper does not cross World boundaries.
You can toggle each of these channels on or off any time you like. In the GUI, use the "Channel Settings" menu near the top of the Chat window. In a MUD client, use "toggletell" and so on. The default for each is on. When you toggle one off, you make yourself deaf to that channel: people can Shout till they turn purple but you won't hear them. If you've toggled off Ask, Page or Tell, would-be conversationalists are told you're not listening; this way they don't keep trying to no purpose.