Ausubel's theory is not particularly in vogue today, perhaps because he seems to advocate a fairly passive role for the learner, who receives mainly verbal instruction that has been arranged so as to require a minimal amount of "struggle". Nevertheless, there are some aspects of his theory that I find interesting.
The advance organizer:
This seems to be the most enduring Ausubelian idea, even though it can be tricky to implement. There is a fair amount of intuitive appeal to the idea of epitomizing an idea before trying to teach the details. We've all had the experience of needing to understand the "big picture" before we can make sense of the details. You could think of the advance organizer as Ausubel's notion of howto provide this.
The comparative organizer:
How do we remember concepts and keep them from fading or being lost into higher-level ideas? Ausubel proposed the comparative organizer as a way of enhancing the discriminability of ideas; i.e., permitting one to discriminate a concept from other closely related ones. A comparative organizer allows you to easily see the similarities and differences in a set of related ideas.
According to Ausubel, the purpose of progressive differentiation is to increase the stability and clarity of anchoring ideas. The basic idea here is that, if you're teaching three related topics A, B, and C, rather than teaching all of topic A, then going on to B, etc., you would take a spiral approach. That is, in your first pass through the material, you would teach the "big" ideas (i.e., those highest in the hierarchy) in all three topics, then on successive passes you would begin to elaborate the details. Along the way you would point out principles that the three topics had in common, and things that differentiated them."