The first category is the sutta pitaka (literally "basket of threads", Sanskrit: sutra pitaka) representing the philosophical teachings of the Buddha and other early teachers, along with some "biographical sketches". The Sutta Pitaka has numerous subdivisions.
The second category, the vinaya pitaka, was the code of ethics to be obeyed by the early sangha, monks and nuns. Some rules and practices were regarded by the Buddha as essential and foundational to the pursuit of his philosophical teachings. Others were invented on a day-to-day basis as the Buddha encountered various behavior problems with the monks.
The third category, the abhidhamma pitaka, is a collection of texts in which the underlying doctrinal principles presented in the Sutta Pitaka are reworked and reorganized into a systematic framework that can be applied to an investigation into the nature of mind and matter. Not much is popularly written about the Abhidharma.
An excellent website on the Pali Canon is http://www.accesstoinsight.org/.
See also Sutra