Scantron is the name of a US company that makes and sells Scantron exam answer sheets and the machines to grade them.
The Scantron system usually takes the form of a "multiple choice, fill-in-the-circle/square/rectangle" form of varying length and width, from single column 50 answer tests, to multiple 8.5" x 11" page forms used in standardized testing such as the SAT and ACT . The forms are sensed optically in a "Scantron Machine" that tabulates and can automatically grade results. Earlier versions were sensed electrically.
Almost anyone who has taken an exam will have experience with Scantron sheets, especially their instructions for use, from "Do not make any stray marks on the answer sheet" and "Fill in each circle/square completely. To change your answer completely erase the mark." to "Please use a number 2 pencil"
A photocopied Scantron form will not work - the green ink becomes too dark, possibly to prevent such copying. It is, however, possible to print forms that will work in the Scantron machine from a personal computer printer, if a properly made grayscale master is available.
A company called GradeMaster makes machines and forms that work with the Scantron system.
