The envelope icon on the Wii's main menu screen brings you into the mail area of the Wii's interface. This isn't exactly a channel, but is a message center.
Here's the main channel interface for the Wii -
The icon in the lower right corner brings you into your mail area. Here's a sample of how that looks.
You click on envelopes to read what they say. Many of these envelopes come automatically from the system based on what games you are playing. For example, when I play with the sports pack, I get messages telling me how well I'm doing and how long I played various games.
If you have multiple profiles, they can send messages to each other - or if you go online, you could send messages to trusted friends. Of course, you're sitting there pointing at one letter at a time, which could take you a while to write your letter!
Wii Channel Information