Any tool that allow you to simply manage content on a web site, without needing to do the HTML coding, is really a CMS.

Blogs packages are really just a sub-set of CMS tools. They are generally less flexible than full CMS systems. But often easier to use.

I would agree with passmark,

Basically to put it to its most basic difference is a blog is mostly more basic than a cms and better suited for one or a small number of contributers and a cms tend to offer more options and addons and are more suited for a larger number of contributers (writers).

Of course that is putting it very bluntly and basic, I am a huge fan of both Word Press (blog) and Joomla (cms). They both have their positives and negatives.

--Tone