An RSS Channel is a container for a number of items of a similar type, meaning that they share the same potential metadata signature.
If the Channel is "Jim's News Stories," then it should only contain news stories by Jim. By trying to place one of Jim's stories into another channel named "Alice's Sports Report" the meaning of the container would be lost.
An RSS channel item would be one of these items in the channel. For example, one of Steve's stories.