Microcontrollers (MCU's) are ubiquitous in our world today. They make up the brain of items like calculators, Palm pilots, cell phones, MP3 players, DVD players, automatic bread makers, vending machines, video game boxes, cars, aircraft, cruse missiles, satellites, mars landers, Furbys, industrial robots and hobby robots. These microcontrollers are single integrated circuit (IC) computer controllers  in which a computer and input output peripherals have been incorporated into a single package. They are then embedded in the various products mentioned above.

The development of embedded computer controllers was motivated by the need for reliability. The first project that employed embedded computer control was the space programs Apollo flight navigation system

"...the Apollo on-board computer system was integrated so fully into the spacecraft that designers called it 'the fourth crew member'. Not only did it have navigation functions, but also system management functions governing the guidance and navigation components. It served as the primary source of timing signals for  spacecraft systems. The Apollo computer system did not have as long a list of responsibilities as later spacecraft computers, but it still handled a large number of tasks and was the object of constant attention from the crew."

By 1971 the worlds first single IC microprocessor was in commercial production. This was the Intel 4004 which soon lead to the 8008. It was the development of the Intel 8008 that lead Bill Gates to the concept that it may be possible to build a personal computer. 

From page 13 of The Road Ahead by Bill Gates c1996

"One summer day in 1972, when I was sixteen and Paul was nineteen, he showed me a ten-paragraph article buried on page 143 of Electronics magazine. It announced that a young company called Intel had released a microprocessor chip they'd named the 8008."

"A microprocessor is a sinple chip that contains the entire brain of a computer. Paul and I realized that this first microprocessor was very limited, but he was sure that the chips would get more powerful, that computers on a chip would improve very rapidly. This insight of Paul's was the cornerstone of all that we did together later, including the founding of Microsoft"

Around 1975 General Instruments produced the first PIC (programable intellegent computer) to be used in conjunction with a 16 bit microprocessor which lacked IO capabilities. Eventually Arizona Microchip Technology was spun off from General Instruments and the PIC was further developed as a stand alone MCU.

from (page 4) PIC1650 datasheet from 1977 image of 1977 a datasheet of a PIC1650

MCU's are typically programmed in either assembly or in C language (although there are now also instances where a basic interpreter is used).