Objective-C was the main programming language used by Apple for its OS X (now macOS) and iOS operating systems and their respective application programming interfaces (APIs), Cocoa and Cocoa Touch, until Swift was introduced in 2014.

Guido van Rossum, a Dutch programmer, began developing Python in the late 1980s. He designed Python with a strong emphasis on code readability, employing significant whitespace and a clear, concise syntax. This design philosophy aimed to allow developers to express concepts in fewer lines of code compared to more verbose languages like C++ or Java. Larry Wall is known for creating Perl, James Gosling for Java, and Yukihiro Matsumoto for Ruby, making Guido van Rossum the correct answer for the creator of Python.

The Xerox Alto, developed at Xerox PARC in 1973, was a landmark in computer history primarily because it was the first computer to feature a graphical user interface (GUI) driven by a mouse. Although not a commercial product, its innovations significantly influenced subsequent personal computer development, particularly the Apple Macintosh. While microprocessors (like the Intel 4004 in 1971) existed before the Alto, and high-level programming languages were also well-established, and removable magnetic storage had been around for decades, the Alto's unique combination of a bitmapped display, windows, icons, and mouse interaction set a new standard for user interaction and laid the foundation for modern computing interfaces.

ENIAC (Electronic Numerical Integrator and Computer), built at the University of Pennsylvania, was the first electronic general-purpose computer. It was Turing-complete and able to be reprogrammed to solve a wide range of computing problems.

While Xerox's Palo Alto Research Center (PARC) developed pioneering GUI technology with systems like the Alto (1973) and the commercial STAR workstation (1981), these were primarily research machines or high-end business systems that were not commercially successful personal computers in the mainstream sense. Apple introduced the Lisa in 1983, which was the first personal computer sold to the general public with a mouse-driven GUI. Although the Lisa was commercially unsuccessful due to its high price, it was quickly followed by the significantly more affordable and popular Macintosh in 1984. The Macintosh truly popularized the GUI and mouse interaction for personal computing, making Apple the company credited with introducing the first *commercially successful* GUI-based personal computer. IBM's PC (1981) initially used a text-based interface, and Microsoft's Windows 1.0 (1985) was an operating environment that ran on top of MS-DOS, appearing after Apple's groundbreaking GUI machines.

SMTP is an Internet standard communication protocol for electronic mail transmission. Mail servers and other mail transfer agents use SMTP to send and receive mail messages.

Before the dominance of MS-DOS in the IBM PC era, CP/M (Control Program for Microcomputers) was the leading operating system for 8-bit microcomputers. Developed by Gary Kildall of Digital Research in the mid-1970s, CP/M was extensively used on a variety of machines, including those based on the Intel 8080 and Zilog Z80 processors. Its design principles, such as a command-line interface and hierarchical file system, significantly influenced later operating systems, most notably Microsoft's MS-DOS. IBM initially approached Digital Research for an operating system for their PC, but negotiations failed, leading them to acquire 86-DOS (a CP/M clone) and rebrand it as PC-DOS, which Microsoft marketed as MS-DOS.

VisiCalc, released in 1979, holds the distinction of being the first major electronic spreadsheet program. Its intuitive interface for creating and manipulating financial models and calculations was revolutionary. For many businesses, VisiCalc's utility alone justified the significant investment in an Apple II personal computer, effectively transforming the PC from a niche hobbyist device into an indispensable business tool. This impact earned it the 'killer app' title, greatly accelerating the adoption of personal computers in professional environments. Lotus 1-2-3, Microsoft Excel, and Quattro Pro all emerged later, building upon VisiCalc's foundational concept.

The C programming language is widely used for embedded systems because it provides low-level access to memory, has a small runtime footprint, and is extremely efficient, which are critical requirements for resource-constrained devices.

In 1981, IBM (International Business Machines Corporation) introduced its first personal computer, the IBM Personal Computer 5150. While other companies had released computers for home use prior to this (e.g., Apple II, Commodore PET), the IBM PC's launch was a watershed moment. It established open standards, fostered a third-party software and hardware ecosystem, and gave the budding personal computer market significant corporate legitimacy, thereby accelerating its adoption and setting a de facto standard for future PC architecture.