K2 was built as a complete replacement of the default article system in Joomla!. Install it like any Joomla! extension, import your articles from the default Joomla! article system and you instantly get a host of new features for your existing content: rich content forms for items (think of Joomla! articles with additional fields for article images, videos, podcasts & other audio files, image galleries and attachments)
While J 2.5 articles have more features, they do not include all of the above. K2 with sigpro and allvideos by JoomlaWorks continues to offer all the above in one package, only purchase necessary is sigpro, if you need galleries