The progenitor was the Apple I, which was a hand-built machine sold to hobbyists.
The first large-scale production computer was the Apple II. It became popular with home users, as well as occasionally being sold to business users, particularly after the VisiCalc spreadsheet was released. See the Computing timeline for dates of machine releases, etc.
The Apple II came with a MOS Technologies 6502 microprocessor running at 1 MHz, 16 KB of RAM, a tape cassette interface and the "Integer BASIC" programming language built into the ROMs. The video controller displayed 24 lines by 40 columns of upper-case-only text on the screen, with NTSC composite video output for display on a monitor or TV by way of an RF-modulator. Users could save and retrieve programs and data on audio cassettes; other languages, games, applications and other software were available on audio cassette too.
Later, an external 5.25" floppy disk drive and controller card that plugged into one of the computer's slots enabled much more convenient data storage and retrieval. The disk drive interface created by Steve Wozniak is still regarded as an engineering design marvel. The controller card had very little intelligence, which made it easy for proprietary software developers to make the media on which their applications shipped hard to copy by using tricks such as changing the low-level sector format or even stepping the drive's head between the tracks; however, other groups eventually sold software such as Copy II Plus that could make backup copies of software under different styles of copy restrictions.
Wozniak's open design and the Apple's multiple expansion slots permitted a wide variety of third-party devices to expand the capabilities of the machine, particularly the Z80 card which permitted the Apple to switch to the Z80 processor and run a multitude of programs developed under the CP/M operating system such as the database dBase II and the WordStar word processing program.
It was followed by the Apple II Plus, which came with the Applesoft BASIC programming language (which added support for floating-point arithmetic but sacrificed integer performance in the process) and had a total of 48 kilobytes of RAM, expandable to 64 KB through a "language card" that let users quickly switch between "INT" and "FP" (Applesoft) dialects of BASIC (but destroying any unsaved program in the process). Addition of the language card also enabled the use of UCSD Pascal, which was released for the Apple at that time.
This was followed with the Apple IIe, which displayed both upper and lowercase letters, supported a high resolution (80 column) text mode, and had 64 KB of RAM expandable to 128 KB. Apple later produced an Apple IIc computer with onboard controllers for common devices such as disk drives, modems, etc., that previously required adapter cards. Along with the Apple IIc, Apple produced an enhanced IIe (identified by its numeric keypad) that used the new 65C02 processor. Apple later developed methods to overclock a IIc safely and produced the Apple IIc+.
The final member of the line was the Apple IIGS computer, released in 1986 alongside the Macintosh SE computer. The IIGS featured a 65C816 processor with 16-bit registers, larger address space with more memory, better color, more peripherals (switchable between IIe-style card slots and IIc-style onboard controllers), and a user interface derived from Mac OS. Apple continued to sell and support the IIGS for a few years after the introduction of the Macintosh line, mainly due to its use in schools.
When Macintosh computers became powerful enough to emulate an Apple II computer, Apple began to phase out the II series in favor of Macintosh computers. This started with the Apple IIe Card that fit into one of the slots on the Macintosh LC computer and connected to a 5-1/4 inch floppy drive. Nowadays, even a PC running Microsoft Windows can emulate certain Apple II models with emulator software such as AppleWin by copying the disk through a serial line; however, emulators cannot run software on copy-restricted media unless somebody "cracks," or removes the copy restrictions from, the software.