Machine instructions. (Many answers are correct; anything symbolic can be represented with bit patterns: graphics, music, floating point numbers, internet locations, video, ...)
A group of 8 bits is a byte. Typically one character is represented with one byte. The agreement by the American Standards Committee on what pattern represents what character is called ASCII. (There are several ways to pronounce "ASCII". Frequently it is pronounced "ásk-ee"). Most microcomputers and many mainframes follow this standard.
When a printer built to print ASCII receives the ASCII pattern for "A" (along with some control signals), it prints an "A". Printers built to other specifications (typically for mainframe computers) might print some completely different character if sent the same pattern.
Most modern printers are much more complicated than the one illustrated on the right. Typically, a modern printer is sent an entire file of information at once. The file describes the layout and contents of an entire page. ASCII characters are just some of the information in the file.
(Thought Question: ) Does the DOS command TYPE expect a file containing ASCII characters?