A Turing machine is a theoretical representation of a computer, as formulated by Alan Turing, the founding father of modern computer science. In a Turing machine, the data (including, in some cases, the encoding of the program) are represented by symbols on a potentially infinite tape. The tape is read and written by a hypothetical "state machine".
Turing machines are an artifice for theoretical discussions of such issues as computability, universality, and the so-called halting problem.
Turing machines are never actually built, except possibly as jokes.