A channel is the medium through which drops are delivered into a river. A river can therefore use more than one channel for fetching drops. If we were to use the analogy of a bathtub, a channel would be analogous to a tap (e.g. the cold water tap) which you turn on/off to fill up the tub with water of a certain temperature.
The following are examples of channels (the supported channels are in bold):
- RSS
- SMS
- Flickr
- YouTube
- Google+