The Tipitaka (Pali ti, "three," + pitaka, "baskets"), or Pali Canon, is the collection of primary Pali language texts which form the doctrinal foundation of Theravada Buddhism. The Tipitaka and the paracanonical Pali texts (commentaries, chronicles, etc.) together constitute the complete body of classical Theravada texts.
The Pali Canon is a vast body of literature: in English translation the texts add up to thousands of printed pages. Most (but not all) of the Canon has already been published in English over the years. Although only a small fraction of these texts are available on this website, this collection can be a good place to start.
The collection of texts concerning the rules of conduct governing the daily affairs within the Sangha — the community of bhikkhus (ordained monks) and bhikkhunis (ordained nuns). Far more than merely a list of rules, the Vinaya Pitaka also includes the stories behind the origin of each rule, providing a detailed account of the Buddha's solution to the question of how to maintain communal harmony within a large and diverse spiritual community.
The "Long" Discourses (Pali digha = "long") consists of 34 suttas, including the longest ones in the Canon. The subject matter of these suttas ranges widely, from colorful folkloric accounts of the beings inhabiting the deva worlds to down-to-earth practical meditation instructions, and everything in between. Recent scholarship suggests that a distinguishing trait of the Digha Nikaya may be that it was "intended for the purpose of propaganda, to attract converts to the new religion."
The "Middle-length" Discourses (Pali majjhima = "middle") consists of 152 suttas of varying length. These range from some of the most profound and difficult suttas in the Canon to engaging stories full of human pathos and drama that illustrate important principles of the law of kamma.