A computer is any machine or device that can be programmed with a set of instructions. Most computers with a CPU, such as smartphones and PCs, can be described as general purpose machines because they can be used for many different things. Software enables a computer to be a games machine, web browser, word processor and perform a wide range of functions.
Devices such as the first Sega Mega Drive (released 1990) could be described as special purpose machines as they were designed to just do one thing – play games.
The first programmable general purpose computer, the Analytical Engine, was designed by Charles Babbage and Ada Lovelace in 1837.
In 1935, British scientist Alan Turing came up with a design for how modern computers would work - with programs to perform many different functions.
The first person to actually build a working, programmable computer was a German inventor, Konrad Zuse. He built the Z3 in 1941. Its program code and data were stored on punched film, and the computer was used to analyse wing flutter on aircraft during the Second World War.
Another early computer was also built at this time. The Colossus - designed and built by engineer Tommy Flowers at Bletchley Park - was made to process large amounts of data, and it enabled British cryptographers to break coded German communication messages.
Processing data manually is extremely time-consuming, and so the invention of the Colossus helped to speed up the process.
The invention of the transistor changed the design of the CPU. Over the years, CPUs have become faster and smaller - but they still retain the same basic function. CPU speeds have doubled every two years or so over the past 50 years. This is in keeping with Moore's Law.