This is Bryan Alexander's definition:
These sections of the Web break away from the page metaphor. Rather than following the notion of the Web as book, they are predicated on microcontent. Blogs are about posts, not pages. Wikis are streams of conversation, revision, amendment, and truncation. Podcasts are shuttled between Web sites, RSS feeds, and diverse players. These content blocks can be saved, summarized, addressed, copied, quoted, and built into new projects.
ReferencesAlexander, B.(2006) Web 2.0: A New Wave of Innovation for Teaching and Learning? Accessed from: http://connect.educause.edu/apps/er/erm06/erm0621.asp