Turing 101

 "What do [people] say about the Turing machine? It's "the simplest computing device". It's the 'basis for modern computers'. It's 'the theoretical model for the microchips in your laptop'. It's the 'mathematical description of your computer'.  None of those things are true. And they all both over and under-state what Turing really did. 
In terms of modern computers, the Turing machine's contribution to the design of real computers is negligible if not non-existent. But his contribution to our understanding of what computers can do, and our understanding of how mathematics really works -- they're far, far more significant than the architecture of any single machine. 
"The Turing machine isn't a model of real computers. The computer that you're using to read this has absolutely nothing to do with the Turing machine. As a real device, the Turing machine is absolutely terrible. 
"The Turing machine is a mathematical model not of computers, but of computation. That's a really important distinction." -- Mark CC

At this time of tremendous media coverage of Alan Turing's work (100th anniversary of his birth), Mark Chu-Carroll offers an excellent primer on what Turing machines really are… and aren't (longread):

 http://scientopia.org/blogs/goodmath/2012/06/24/turing-machines-what-they-are-what-they-arent/