HTML text and images on a page. Later revisions included some attempts at providing semantic data which allows basically for automated categorization and correlation of HTML documents.
HTML5 actually strives to be something very different. It aims to be more of an application development platform, which includes not only laying out text and images, but also playing video and audio, interactive 2d and 3d graphics, storing data in the application, dealing with online and offline access to data, and real-time networking protocols for exchanging data as it happens.