This article is from the Apple II Csa2 FAQ, by Jeff Hurlburt with numerous contributions by others.
Number of bits usually indicates how big a chunk of data a computer's main microprocessor can manipulate. The Apple IIgs is based on the 65C816 microprocessor and is considered to be a 16-bit machine. Previous Apple ]['s are based upon pure 8-bit microprocessors such as the 6502 and 65C02. These are considered to be 8-bit machines. Sometimes II+ or IIe or IIc software is called "8-bit software". The 65C816 is a member of the 6502 family which includes expanded registers and adds many new commands while retaining the ability to go into 8- bit mode. So; the GS can run most 8-bit wares designed for older Apple II machines as well as newer 16-bit wares. Meanwhile, 8-bit machines are pretty well limited to 8-bit wares. --Rubywand