The story of the development of the Apple 1 has became a "legend". Here's how it goes:
Steve Wozniak, was working for Hewlett-Packard at the time, wanted to build his own computer. He couldn't afford the Intel 8080 CPU (this CPU was very popular then, as it was used in the Altair 8800 & IMSAI 8080, but was pretty 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 (which was pretty compatible with the Motorola 6800).
The computer was easier to use than the Altair: notably, the Apple 1 had a keyboard connector and could display characters on a TV whereas the Altair used LEDs for display. The display rate was very slow, only 60 characters (!) per second.
Steve Jobs, who programmed the game "Breakout" for Atari (with a little help from "the Woz"), was interested in this computer. Together they created the Apple Computer Company and tried to sell their computer. Paul Terrell, the owner of a computer shop, was interested in this computer, but fully assembled (the Apple 1 was sold "naked", no monitor, no power supply, no keyboard, no tape drive, etc.) and with a cassette interface, which Wozniak designed. He sold it with the Basic he wrote soon after.
They sold about two hundred 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.
The nowadays mythic Apple 1 was followed the next year by the Apple ][.