How Orchard works
I just finished writing a long documentation topic on the Orchard project wiki that aims at being a good starting point for developers who want to understand the architecture, structure and general philosophy behind the Orchard CMS.
It is not required reading for anyone who only wants to write Orchard modules and themes but hopefully it will help people who want to evaluate the platform and start writing patches.
Read it here:
http://orchardproject.net/docs/How-Orchard-works.ashx