Final answer:
Platform as a Service (PaaS) is the utility computing model where clients are provided with operating systems and supporting software by vendors, and write their own applications. It allows for more focused software development without infrastructure management concerns.
Step-by-step explanation:
The variant of utility computing where vendors provide the operating system and supporting software like database management systems, but client firms write their own code, is referred to as Platform as a Service (PaaS). With PaaS, developers can focus on building their unique applications without worrying about underlying infrastructure management. This enables faster development and deployment cycles compared to managing hardware and operating systems.
The history of computing shows a battle between free, open resources and commercial efforts. Early operating systems were proprietary, such as IBM's AIX, Microsoft's Xenix, Sun Microsystems' Solaris, and SGI's IRIX, which were all variations stemming from the UNIX system that later became commercialized.