Can someone please help me understand what "Content Items" are?
I found a lot of experts who recommend incorporating them in your modules. However, I am not clear why it is necessary. Can someone please tell me what are the pros and cons for the same?
Thanks in advance.

A content item, simply put, is a piece of content on your site. This content can be an entire page and/or it can be an individual module on a page. Something like a forum module, which is a single module on a single page, contains multiple content items per module instance. This thread, for example, is one content item and every other thread is its own content item as well. Outside of the core, content items within modules are only available if the module developer integrated it (ie. I integrated content items into the forum module). 

As Sebastian mentioned, once a module is integrated with content items it can then expose other functionality such as tagging and categorization (folksonomy and taxonomy) which are related to content items at the core level. In the future, expect to see other controls and functionality available for use with content items (which is why I am stressing the importance of integration for module developers). 

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Chris Paterra

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