Content Management System (CMS) is a general-purpose system that allows registered authors to upload and manage all types of content - text articles, images, videos, audio files, etc.
CMS will also allow site administrators to determine exactly what each page looks like: layout, placement of navigation, advertisements, etc. Administrators will also control who is allowed access to the system. The authors in this case will be people within the organization, not the public who visit the site.
The newspaper website is a very good example of a CMS.
The blogging mechanism can be viewed as a type of CMS that has been adapted to simplify the creation of short (ish) articles with additional comments. There will usually be only one author on a blog, although this is not always the case.
There will be (usually) very little control over the layout. Having said that some blogging mechanisms will allow you to control more than others, and may be indistinguishable from content management systems.
A forum is a place where users can post articles, etc., and other users post answers to them. Thus, it can be considered as a specialized type of CMS.
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