A 'Turing Machine' is an elementary model of a Computer. The Machine has a number of 'States' and scans a (potentially infinite) 'Tape' of 'Cells' that contain Symbols from a finite set of such. Depending on the Symbol the Machine encounters, it can a) Change the Symbol in that Cell. b) Change to another 'State'. c) Move either Right or Left, to the next Cell on the 'Tape'.