Apple II was the successor to the Apple 1 on which it was largely based. It was the very first commercial success of the Apple Computer Company.
Because Steve Wozniak wanted to demonstrate his Breakout game with the new Apple II, he decided to add color, sound and minimum paddle support to the Apple 1's heir.
The Apple II came with 4 KB RAM, but it was possible to add 4 KB or 16 KB RAM chips. Thus, the system could have memory in the following sizes: 4K, 8K, 12K,16K, 20K, 24K, 32K, 36K, or a full 48K. This was one of the strong points of the Apple II: from the beginning, it was designed with expansion in mind. The 8 expansion slots were further proof of that - users could expand their system easily, just by plugging cards into the slots.
The ROM included the monitor, a 6502 disassembler, a mini-assembler called 'Sweet 16' and the Integer Basic written by Wozniak in machine language, assembled by hand on paper! Not having to load a language from tape or disk to start programming was also a significant advantage over competitors. Even the innovative plastic case proved to be an important feature to attract customers.
Just a few months after the Apple II presentation at the First West Coast Computer Fair in April 1977, Apple received about three hundred orders for the Apple II, over a hundred more than the total number of Apple-1's sold.
The Apple II was followed in 1979 by the Apple II+, which brought some enhancements.