choster - 3:08 pm on Mar 27, 2012 (gmt 0)
It really depends on what you are trying to do and how your site has been implemented. Broadly speaking, "content" is the informational material on any website, as opposed to its navigational/utilitarian or decorative elements.
In theory, such material in Drupal is traditionally stored in a node, and any "content" modules are designed to work first with the node system. Content access permissions, content control modules, and CCK (the Content Construction Kit) applied to nodes and nodes alone. Taxonomy terms, files, and comments are separate elements which exist to extend the node system.
In practice, many a lazy site builder will put content into blocks, Views headers, footer messages, taxonomy vocabulary descriptions, and anywhere else they see a textarea field. I would call this bad practice— such areas are not included in search results, content permissions systems, workflow systems, and the like, and may thus pose usability or security risks— but that is the state of things.
In Drupal 7, as ergophobe noted, the concept is expanded, in part because the Entities concept has been introduced, but also because a website is no longer a collection of pages viewed in a desktop web browser: it can provide feeds, snippets delivered through services, and various other outputs. So while browsing the "Content" menu will still return a list of nodes alone, it is possible to insert editable content into other things without making them into nodes. Some Drupal 7 entities may be very node-like and could be considered content, whereas others could remain very auxiliary.