ENIAC I (Electronic Numerical Integrator And Computer) was the first all electronic digital computer (that is, it used the properties of electrons to achieve its results, different from the mechanical computers put together by the likes of Wilhelm Schickard, Blaise Pascal, and attempted by the famous Charles Babbage).
It was developed and built by the U.S. Army for their Ballistics Research Laboratory with the purpose of calculating ballistic firing tables. ENIAC was designed by J. Presper Eckert and John William Mauchly of the University of Pennsylvania. The computer was commissioned in May 1943 as Project PX and it was constructed at the Moore School of Electrical Engineering from mid-1944, it was formally operational from February 1946 having cost almost $500,000. It was transferred to the Aberdeen Proving Grounds, Maryland in 1947.
ENIAC was a decimal machine using ten position Ring counters to store digits. Arithmetic was performed by "counting" pulses with the Ring counters and generating carry pulses if the counter "wrapped around". The idea being to emulate in electronics the operation of the digit wheels of a mechanical adding machine. Initially ENIAC could perform 5,000 simple addition operations every second, although with division it could only manage 38 operations per second.
Physically ENIAC was a monster - it contained over 18,000 vacuum tubes, 70,000 resistors, 10,000 capacitors and around 5 million hand-soldered joins. It weighed 30 tons, was roughly 2.4 m by 0.9 m by 30.5 m and took up 167 m² and consumed 160 KW of power.
Eckert and Mauchly took the experience they gained and founded the Eckert-Mauchly Computer Corporation, producing their first computer, BINAC, in 1949 before being acquired by Remington Rand in 1950 and renamed as their UNIVAC division.
ENIAC ran until October 1955. It was a one-off design and was never repeated, the freeze on design in 1943 meant that the computer had a number of short-comings which were not solved, notably the lack of ability to store a program. But the ideas generated from the work and the impact it had on people such as John von Neumann was profoundly influential in the development of later computers, initially EDVAC, EDSAC and SEAC. A number of improvements were also made to ENIAC from 1948.
See also: Manchester Mark I, Colossus computer, and Z3.