The below topics are the core of what an Orchard developer should know. Other, less important items are listed under Extended APIs.
This page and many others here are part of the open-source, portable package of Orchard Core goodies: the Orchard Dojo Library for Orchard Core. The Library is freely accessible by anyone; just go to its repository to download its full content (you can even download the textual content of the Library as a big concatenated document in HTML), file issues or fork it! You can even edit it online on Bitbucket, just click on the Edit link in the upper right corner!
Note that the Library also has an Orchard 1.x version, check it out here!