During the late 1960s, Bill Gates and his school friend Paul Allen learnt the BASIC programming language whilst studying at Lakeside School in Seattle.
BASIC's popularity stemmed from the fact that it was simple to use for beginners, but had functionality that could be stretched by advanced coders.
While BASIC was designed in 1963, it wasn't until Gates and Allen produced a version for the Altair 8800 that it really started to fly. In 1975, the two sold the code to MITS for $3,000.
BASIC sparked the home computing boom of the 1980s, appearing on the IBM PC, Commodore VIC-20, BBC Micro and the Apple II (to name but a few).
2. Commercialising the operating system
Gates and Allen founded Microsoft in 1976. In 1980, the company inked a landmark deal with IBM to produce an operating system for its up-and-coming personal computer (PC). Microsoft bought 86-DOS from Seattle Computer Products, a Disk Operating System designed for 8086-based computers.
It used the 86-DOS code as the basis for IBM's first PC OS, which became known as PC-DOS. When clones of IBM's PC started to appear, Microsoft pushed their own version of PC-DOS, MS-DOS, to system builders.
3. Launching Windows 95
When you look back at the career of Bill Gates, it's easy to focus on Microsoft's biggest product - Windows. You can now find versions of Windows on around 90 per cent of the world's computers.
It powers business servers, mobile phones, PDAs, cash machines, retail displays, set-top boxes and old Sega Dreamcast consoles. Windows is everywhere. Windows 95 was arguably the game-changer.
The original Windows 1.0 OS was a effectively a graphical version of MS-DOS, born to compete against the GUI on Apple's 'Lisa'. In comparison, Windows 95 was designed to replace MS-DOS and enabled revolutionary features such as multi-tasking and Plug and Play.
4. Becoming the richest man in the world
Bill Gates became the richest man on the planet at the age of 39. He stayed at number one on the Forbes list of the World's Richest People until 2007.
According to Wikipedia, Bill Gates was worth more than $101 billion in 1999; while Wired reports that, when Microsoft stock hit a high in 1996, Gates was earning $30 million a day.
He's now third on the Forbes list (behind his friend Warren Buffet and Mexican telecom mogul Carlos Slim Helu) with an estimated fortune of $58 billion.
5. Giving his money away
As Bill Gates clocks out of his day job at Microsoft, the world will arguably be a better place for it. Inspired by the work of John D Rockefeller, Gates and his wife Melinda head up the charitable behemoth that is the Bill & Melinda Gates Foundation.
In 2006, Warren Buffet gifted $31 billion to the Foundation, which already had over $30 billion of Gates' own money in its coffers. In 2007 alone, the Foundation spent over $2 billion on global education and health initiatives.