The Macintosh; branded as Mac since 1998, is a series of personal computers (PCs) designed, developed, and marketed by Apple Inc. Steve Jobs introduced the original Macintosh computer on January 24, 1984. This was the first mass-market personal computer featuring an integral graphical user interface and mouse. This first model was later renamed to "Macintosh 128k" for uniqueness amongst a populous family of subsequently updated models which are also based on Apple's same proprietary architecture. Since 1998, Apple has largely phased out the Macintosh name in favor of "Mac", though the product family has been nicknamed "Mac" or "the Mac" since the development of the first model.
The Macintosh, however, was expensive, which hindered its ability to be competitive in a market already dominated by the Commodore 64 for consumers, as well as the IBM Personal Computer and its accompanying clone market for businesses. Macintosh systems still found success in education and desktop publishing and kept Apple as the second-largest PC manufacturer for the next decade. In the 1990s, improvements in the rival Wintel platform, notably with the introduction of Windows 3.0, then Windows 95, gradually took market share from the more expensive Macintosh systems. The performance advantage of 68000-based Macintosh systems was eroded by Intel's Pentium, and in 1994 Apple was relegated to third place as Compaq became the top PC manufacturer. Even after a transition to the superior PowerPC-based Power Macintosh line in 1994, the falling prices of commodity PC components and the release of Windows 95 saw the Macintosh user base decline.
In 1998, after the return of Steve Jobs, Apple consolidated its multiple consumer-level desktop models into the all-in-one iMac G3, which became a commercial success and revitalized the brand.
Basilisk II is an open source emulator of 68xxx-based Macintosh computers for Windows, OS X and Linux. With Basilisk II, one can boot Mac OS versions 7.x through 8.1. Users require a Macintosh ROM image and a copy of Mac OS to use with the emulator. You can find all info on this Emaculation page and download the latest version here, where You can also find links to tutorials.
SheepShaver is an open source PowerPC Apple Macintosh emulator. Using SheepShaver (along with the appropriate ROM image) it is possible to emulate a PowerPC Macintosh computer capable of running Mac OS 7.5.2 through 9.0.4. SheepShaver is considered a good replacement for the Classic Environment which is not available in the most recent versions of Mac OS X. You can find the latest version, infos and tutorials on this Emaculation page.
The Mini vMac emulator collection allows modern computers to run software made for early Macintosh computers, the computers that Apple sold from 1984 to 1996 based upon Motorola's 680x0 microprocessors. All info can be found on Mini vMac homepage where You can also download the latest version.
OS X can be virtualized on modern intel Macs with help of virtualization software. Some hacks are required as Apple posed restrictions on running some guest versions of OS X on OS X hosts...