In SharePoint 2003, SPS and WSS, you were limited to a single set of columns and metadata tags per list or document library. This meant that all of the items that were in those lists and libraries were tied to that schema. To solve this problem, SharePoint 2007 introduces the content types.
Content types are a way to manage the metadata and behaviors of a document, list item, or folder in a centralized, reusable way. At their core, content types contain a set of properties, a document template, a set of policies, and any applicable workflows. Site content types created on the site can be reused in multiple lists, and they are visible in that site and all child sites. Content types can store custom XML data, allowing for the implementation of custom solutions. MOSS 2007 doesn’t care what file format or type of media you are working with. In other words, Office files, PDFs, TIFFs, e-mail, videos, and physical records can all be classified using content types.
Site columns work nicely with content types as a way to standardize the metadata that is captured when a particular content type is selected. Both site columns and content types are created at the site level and then copied locally as customizable instances or list content types and list Columns when you add them to a list. When a site column is added to a content type, the content type actually references that site column definition.
Content types can include the following:
Document type: Word, PowerPoint, ...