The story of the development of the Apple I is well known and has became a "legend".
Steve Wozniak, who was working for Hewlett-Packard at the time, wanted to build his own computer. He couldn't afford
the Intel 8080 CPU, which was very popular, as it was used in the Altair 8800 and
IMSAI 8080, but was very expensive. He would have used the Motorola 6800 but it was also much too expensive.
Finally he decided to build his computer around the MOS 6502 CPU, which was pretty compatible with the Motorola 6800.
Apple I was easier to use than the Altair notably, due to the ability to connect a keyboard
and video display, wich allowed displaying the characters rather than interpreting LEDs on the Altair's front panel.
The display rate was very low, only 60 characters per second, but that was not a problem.
Steve Jobs, who programmed the game "Breakout" for Atari (with a little help from "the Woz"), became
interested in this computer. Together, in April 1976, they created the Apple Computer Company and started selling Apple I for $666.66 each.
In one year they sold about 200 of these units. This machine was so popular that Jack Tramiel of Commodore offered
to buy Apple. Apple was, at the time, a major purchaser of MOS 6502 processors and Commodore owned MOS Technologies.
Wozniak wanted $15,000 more than Tramiel offered. Needless to say, the deal fell through.
In April of 1977 Apple II was releasd, which led to discontinuation of Apple I in October of 1977.