Structured Authoring Survey from Scriptorium
Sarah defines authoring as "a publishing workflow that lets you define and automatically enforce consistent organization of information." Typical structured authoring models include DITA, DocBook, S1000D, ATA, and SPL. Flare, Robohelp, wikis, and other help authoring tools and platforms that do not enforce a structure aren't typically considered structured authoring.
If you've been following my blog for a while, you may remember my brief interview with Sarah from the 2009 STC Summit.