What kind of input device is necessary to input bar code information to the computer?

There are adopted standards for how information is presented in a barcode form. Which basically means a bunch of folks decided on a universal set of characters to depict letters and number using a series of stripes of different widths for each letter and number. Code128 is the most widely accepted of these fonts.

In order to read a barcode you need a barcode reader which is nothing more than a low density laser that when aimed at a barcode, simply measures the difference in the beam as it bounces back and reports the resulting measurements to a built in set of instructions which in turn sends back characters to the computer as if it was a string of keys being pressed on a keyboard.