Paul M.B. Vitanyi's "Turing Machine"
This article is fairly challenging; read through it to the best of your abilities for a detailed description of the Turing Machine. After you have read this article, you should know how to define Turing machine and summarize the Church-Turing theses. Make sure you know what the Halting problem is.
Click https://web.archive.org/web/20160703014054/http://www.scholarpedia.org/article/Turing_machine link to open resource.