What are the origins of the control and alt keys?

Crtl and alt have become common place on just about any computer keyboard sold anywhere in the world (I even saw a picture of them on a typewriter...). But what were these sacred modifier keys first used for? What's the difference between them and other keys used as modifiers? How has their used evolved into what it is today?

Short answer:

Ctrl stands for "Control Key". It was originally used to send control characters to terminals.
Alt stands for "Alternate Key". It's named so because it enables alternate uses for other keys.

Long answer: See Wikipedia:

Control Key

Alt Key

Ctrl goes way back to the earliest days of computing and was used for entering ASCII control characters on an ASR33 teletype machine (similar to the ones you see in movies in the newsroom that make so much noise). They could run at the blinding speed of 110 baud (or roughly 10 characters/second).

As far as the Alt key goes, I first saw it on the original IBM PC. It's primary use then was the "three finger salute," or Ctrl+Alt+Delete.

A couple of interesting, and relevant articles with respect to control characters:

Control Character

Teleprinter

ASCII

The "Control" part comes from "carriage control". The platten (round rubber cylinder that keys bounce on) moves back and forth and up and down on a "carriage" in a typewriter and the teletype mimics a typewriter. However, in a typical teletype the platten only rotates and the print head only moves back and forth. But in those days you got so tired pushing the KSR-33 teletype keys down that any abuse of the English language was long forgotten.