Turing Machiene

"A Turing machine is a device that manipulates symbols on a strip of tape according to a table of rules. Despite its simplicity, a Turing machine can be adapted to simulate the logic of any computer algorithm, and is particularly useful in explaining the functions of a CPU inside a computer."                                                                                                          

In its simplest form, a Turing machine is composed of a "tape", a ribbon of paper of indefinite length. There is a "head" that can read the symbol, chose to write a new symbol in place, and then move left or right. The Turing machine is said to be in a certain "state". Finally, the program is a list of "transitions", that is a list that says, given a current state and a symbol currently under the head, what should be written on the tape, what state the machine should go, and whether the head should move left or right.
The tape is used to store data. In addition, it can also store a series of transitions (a small programs) and thus, the head can run "sub-programs". 

By analogy with modern computers, the tape is the memory and the head is the microprocessor. 
Although it is composed of pretty simple capabilities, Turing argued that this simple machine could performed any computation, that is, could realize anything that results from operations.