Mac OS X has, at different times, been associated with several different names. At one point it was called Rhapsody. Prior to that, it was NeXT's OpenStep and NeXTStep platform. The underlying Unix guts were also released as an open source project, Darwin, which includes BSD and the Mach kernel. With that in mind, explaining where Mac OS X started and where it is now will contextualize Mac OS X in its current incarnation.
Mac OS X 10.0 was the first commercial release of Mac OS X. That release, however, wasn't particularly usable.
Convincing a large body of developers to embrace a new platform is not easy. You can release developer seeds, betas, and prereleases all you want, but at the end of the day, major operating system vendors have to release something that can be called a 1.0 product (or, in the case of Mac OS X, a 10.0 product). Releasing this product lets users know that you're transitioning from testing to "prime time."
The commercial release of Mac OS X 10.0 was just that: it was Apple's way of telling developers that the system was ready to go and that they should get on board. At this point, Apple began shipping Mac OS X 10.0 with their hardware, but didn't make it the default operating system. The release was lacking in quality, features, and supported applications, and everyone knew that the product needed more work.
Mac OS X 10.1 marks what most people consider the first usable version of Mac OS X. Developers fixed a lot of important bugs, addressed performance issues, and added missing features.
Even more significant, however, was an Apple announcement at Macworld in January 2002. During one of the conference's keynote addresses, Steve Jobs announced that Apple would begin shipping Mac OS X as the default operating system. Users could still switch back to Mac OS 9 if they wanted, but when someone took that shiny new iMac out of the box, Mac OS X's Aqua greeted them. Apple's commitment to Mac OS X as their default platform was a clear message?developers and users both were assured of Apple's commitment to Mac OS X as an operating system for mainstream use.
A few patches quickly followed the 10.1 release. Mac OS X 10.1.1 became Mac OS X 10.1.4. More importantly, a large number of critical applications became available, such as Microsoft Office and Adobe Photoshop. For developers, a large number of open source projects started to make regular binary builds available for the Mac OS X platform. Are you interested in MySQL, Apache, PHP, or Tomcat? All are now available, prebuilt specifically for Mac OS X. Some open source projects (such as PostgreSQL) that weren't even available for Windows have become available for Mac OS X.
Then Macromedia announced that their MX line (products like Flash, Dreamweaver, and Fireworks) were to be made Mac OS X-native via Carbon. Suddenly, the best platform for Unix and web application development started to resemble Mac OS X. Furthermore, several Java applications became available for Mac OS X. Many were server applications or developer products, but their appearance started to convince users that Mac OS X was becoming a friendly platform for developers.
This release, despite being the first major release to not offer upgrade pricing, was in many ways a major infrastructural improvement. Much of the technology included in this release, such as Rendezvous (Mac OS X's autoconfigurable networking), had a distinctly infrastructural feel. Most significantly, this release included several low-level improvements required for Apple's JDK 1.4 implementation. Although Jaguar shipped with a JDK 1.3 implementation, JDK 1.4 can be installed on Jaguar.
Future releases of Mac OS X will ship with JDK 1.4 support (or whatever the latest JDK version is at release time). As of this writing, the contents of the J2SE 1.5 release are already under discussion for inclusion in Panther, the code name for what will most likely be Mac OS X 10.3.