The channel refers to the medium by which an article is available. Salesforce Knowledge offers four channels:

• The internal app. Salesforce.com users can access articles in the Articles tab depending on their role visibility.
• The Customer Portal. Customers can access articles if the Articles tab is available in the portal. Customer Portal users inherit the role visibility of the manager on the account.
• The partner portal. Partners can access articles if the Articles tab is available in the portal. Partner portal users inherit the role visibility of the manager on the account.
• The public knowledge base. Articles can be made available to anonymous users by creating a public knowledge base using the Sample Public Knowledge Base for Salesforce Knowledge app from the AppExchange. Creating a public knowledge base requires Sites and Visualforce.