"This presentation examines various extensions to the standard Turing machine model and establishes their equivalence to the standard model. These extensions include the multi-tape, nondeterministic, and oracle Turing machines. Before we get into those extensions, let's review the basic concept of Turing machine.
"What is a TURING MACHINE?"
"In 1936 Alan Turing, a British Mathematician, came up with an idea for an imaginary machine that could carry out all kinds of computations on numbers and symbols. A Turing machine is an abstract representation of a computing device. The Turing Machine consists of
"The Input/Output Tape is divided into cells. The cells contain the input and output symbols and change frequently as a program is running. The standard TM usually contains only a single tape.
"The Turing Machine consists of a read/write head that scans the Input/Output Tape. Computation begins with the machine, in a given "state", scanning a cell. If a binary machine is considered, it erases what it finds there, prints a 0 or 1, moves to an adjacent cell, and goes into a new state. This behavior is completely determined by three parameters:
"The table of instructions specifies, for each state and binary input, what the machine should write, which direction it should move in, and which state it should go into. (E.g., "If in State 1 scanning a 0: print 1, move left, and go into State 3".) The table can list only finitely many states, each of which becomes implicitly defined by the role it plays in the table of instructions. These states are often referred to as the "functional states" of the machine.
"A Turing machine, therefore, is more like a computer program (software) than a computer (hardware). Any given Turing machine can be realized or implemented on an infinite number of different physical computing devices. Computer scientists and logicians have shown that Turing machines -- given enough time and tape -- can compute any function that any conventional digital computers can compute."
Quoted from Sheng Wen http://www.geocities.com/s2swen/Turing_Machine.html