What are AdSense Channels:

Channels are a special tool provided by Googe Adsense. It is integrated in every account and it is free to use. Google Adsense only gives 25 channels in every account.
Channels are very easy to use. It is used to track page impressions of Ads, number of times of Clicks, eCPM, CPM and the revenue earned from that certain Ads.
By using Channels, one can track the performance of each Ads and oe can have a better idea on which type of Ads are interested by their visitors/users.

How To Use Channels?
http://adsense-track.blogspot.com/2007/09/how-to-use-adsense-channels.html