The difference between a blog and a CMS is somewhat difficult to classify as both are dealing with content and the functionalities are getting closer.
Content management system (CMS) is a term to clasify programs that can help you to manage content, a full featured content management system can help you to organize an entire library online if you want to, and these type of CMS are generally used in online publishing or websites that is dealing with lots of articles, books, audio, video etc. that needed a system to organize the content and provide searching capability. Generally these programs are paid license and price tags are on the higher end.
A blog can be view as a subset or rather a branch out from traditional CMS as blogs are more focusing on showcasing or sharing a certain topic of interest by the blog author or thoughts on a subject. The more important function or rather can be view as the main difference between a traditional CMS and a blog is that a blog is providing the one-to-one and one-to-many communication channel that is lacking in the traditional CMS, and this evolution of a CMS is a product that is classify under the term web 2.0 where communication / interaction is the key focus.