What are the sub divisions of the Tripitaka and what do these areas cover?
Since I don't know how deep in the Tipiṭaka's structure you want the answer, I'll try to give the most in-depth answer. I'll present the structure in a tree chart and, beneath it, I'll refer to an article with an in-depth analysis of the canon which provides the summaries. 



Structure

(Click to enlarge)





Summaries

Summaries are provided in "Chapter I: Textual Analysis of the article "An Analysis of the Pāḷi Canon" by Russell Webb. It also contains an excellent index of the canon, and an extensive bibliography.