The knowledge of the below APIs is not necessary for all developers as these are needed only for more special tasks.
There is no need to go into details about all providers, evaluating a few provider types used in Projector and a Token provider is enough.
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!