What's a Turing machine?
What is a Turing machine and why do people keep mentioning it? My IBM PC is all I need to do my computation! Why does anyone care about these machines?
Answer
The reason that Turing Machines are a big deal has to do with the study of classical Computing Science or Theory of Computation type stuff. It's basically about analyzing the general properties of a computer, such as what theoretical abilities and limitations a computer has, as well as what we mean when we talk about "computing" something.
One example of something that one might study using Turing Machines is The Halting Problem. While this problem is something of an academic exercise, it has easily tangible real-world implications. Why not write a debugger that will simply tell you whether or not your program contains any infinite loops? The Halting Problem establishes that solving this problem for the general case is impossible.
The study of Turing Machines also lends itself to studying language grammars and classes of thereof, which leads into programming language development. The term "regular expressions" comes about because they are a regular grammar, and the study of these grammars (part of Theory of Computation) will tell you more about exactly what kinds of problems regular expressions can solve and what they can't. For example, a traditional regular expression syntax won't be able to solve the following problem: parse some number N of 'a' chars in input, and then parse the same number N of char 'b'.
If you're interested in a good text about this sort of thing, check out Introduction to the Theory of Computation by Michael Sipser. It's good.