Batch processing operating system
The first computers used batch processing operating systems. Applications were started and run to completion. Inputs were data files (typically on punched cards or magnetic tape), and outputs were paper reports and data files. These applications kept expensive computers running efficiently, and were wasteful of people's time. (IBSYS, SOS operating systems). IBM dominated the batch processing platform.
Timeshared operating systems
Several users shared the computer at the same time. They interacted directly, without the intervention of a professional operator as they had with batch processing systems. Users of early timesharing systems used printing terminals. When technology improved, terminals with displays replaced printing terminals for most applications. (Various from IBM, Multics, Unix, VMS operating systems). Digital Equipment Corporation, Data General, Hewlett Packard and Control Data thrived during this era. They cut into the IBM market share, and marginalized the other batch processing companies.
Personal computer with a character-oriented user interface
As cost dropped, personal computers began to displace time shared computers. They typically used character-oriented displays in which the computer could be programmed to move the cursor to any location on the screen. (CP/M and MSDOS operating systems). Prior to MSDOS, operating systems were typically proprietary. Since MSDOS ran on many manufacturers hardware, Microsoft, a software company, became dominant. Application software companies like WordPerfect, WordStar, Lotus, and VisiCalc developed dominant "killer applications."
Personal computer with a Graphical user interface
With continued technology improvement, personal computers with GUI operating systems moved out of the research lab into the mainstream. These used bitmapped displays in which the computer could address each pixel on the screen. (Apple OS, Windows operating systems). Microsoft and Apple made the GUI transition, but the dominant applications were replaced by MicroSoft Office.
Local area networks
Stand alone personal computers were tied together forming local area networks (LANs). The LAN allowed for intranet applications in which users could share data stored on common servers. They could also share other resources like printers and FAX machines. Applications often involved a client program running on the user's desktop and a server on the network. (Appleworks, Novell Netware, Windows networks). Novell was the early leader, but was caught and passed by Microsoft.
Wide area networks
As communication speed increased, wide-area TCP/IP networks become the dominant application-development platform. Client-server applications like e-mail, the Web, and file transfer dominated. Open source software become important during this period.
The Internet as a platform
Today we see the rise of second generation network-based applications. Many network databases and applications provide open application programming interfaces (APIs) enabling developers incorporate them into their own applications. You might think of them as client-multi server or composite applications. They are informally called mashups. (Companies like Google, Amazon, Microsoft and many startups are vying for dominance).
The mobile Internet as a platform
The next platform appears to be the mobile Internet. We have innovation in portable form factors, increased mobile Internet access, and Apple is beginning to open the iPhone to developers. Many competitors are vying for dominance in this emerging platform -- Apple, Microsoft, Google, and others -- but Apple fired the first real shot with the iPhone. A year after it's announcement, the iPhone had 71% of the US market for mobile Internet access.
Articles on the Internet as a platform: