Turing and Turing Machines

Turing Background
The Turing Machine

The Church-Turing Thesis
The Universal Turing Machine

The Halting Problem

A Nice Clarifying Case of Deterministic but Unpredictable: Conway's Life

Some references:

Hodges, Andrew (1992) Alan Turing: The Enigma. [Nice biography of the extraordinary man. Highly recommended. Lots of great stuff is also on Hodge's Turing www site.]

Sipers, Michael (1992) Introduction to the Theory of Computation. [A very fine introduction to machine theory, although the first edition to which I refer to above is drowning is typos and slighly more serious errors. Sipser caught these and posted them to his www site, but if a later edition comes available that will be a great buy.]