What is a Turing machine?

There are several ways to define the Turing machine, all of them equivalent. The definition used by gturing is as follows. A Turing machine consists of two parts:

To make the machine work, you must also give it a set of rules, which determine the operations it should perform depending on the current symbol it reads from the tape and the head's state. Typical rule can be "If the symbol read is 1 and the internal state is 0, change the current symbol on the tape for a space, move to the left, and change the internal state to 3". Formal description of the rules can be found in the section called Program File Format. You can think of the set of rules as a "program" for the Turing machine.

If you want to know more about Turing Machines, check this Turing Machine Introduction.