Difference between a Knowledge Base and a CMS?

[SB]

What is the difference between a Knowledge Base and a Content Management
System, if any? Note that Linux support is important.

I am looking at a CMS (Alfresco - based on Documentum -
http://www.alfresco.com/ ) which was highly recommended to me and for which
I read a lot of positive feedback.

Somebody else in my company was talking about a Knowledge Base (Talisma:
http://www.talisma.com/tal_products/knowledgebase.aspx which is apparently a
software created in part by Salesforce which is already used in the company)

Is there a difference between KM and CMS? Can somebody tell me the
difference between the two or send me a pointer where I can compare
(provided that I am not comparing apples and oranges)?

Thank you very much,

Sylvia