ENIAC (Electronic Numerical Integrator And Computer) was the first all-electronic computer (that is, it used thermionic valves rather than electromagnetic switches), although the earlier, lesser-known and electromechanical Z3 by Konrad Zuse was truer to being what we today understand as a digital computer, as Zuse was the first to use the binary system to realize the principles founded by Charles Babbage. ENIAC also differed from earlier calculating devices in that it was designed and used to be Turing-complete - that is, a truly universal computing device - unlike earlier devices (although in 1998 the Z3 was also proven to be Turing-complete).
ENIAC 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 on May 17, 1943 as Project PX, constructed at the Moore School of Electrical Engineering from mid-1944, and formally operational from February 1946 having cost almost $500,000. It was then shut off on November 9, 1946 for a refurbishment and a memory upgrade. ENIAC was unveiled on February 14, 1946 at the University of Pennsylvania and was transferred to the Aberdeen Proving Grounds, Maryland in 1947. There, on July 29th of that year, it was turned on and would be in continuous operation until 1955.
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. Each of ENIAC's twenty ten-digit signed accumulators could perform 5,000 simple addition operations every second (total 100,000 addition operations per second). The ENIAC could only manage 357 multiplication operations per second or 38 division (or square root) 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, took up 167 mē and consumed 160 kW of power. In fact, people holding grocery bags full of vacuum tubes would be strategically stationed to replace burning out tubes.
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 2, 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 inability to store a program. But the ideas generated from the work and the impact it had on people such as John von Neumann were profoundly influential in the development of later computers, initially EDVAC, EDSAC and SEAC. A number of improvements were also made to ENIAC from 1948, including a primitive read-only stored programming mechanism  using the Function Tables as program ROM, an idea proposed by John von Neumann. This modification reduced the speed of ENIAC by a factor of 6 times, but as it also reduced the reprogramming time to hours instead of days, it was considered well worth the loss of performance.
Today, a chip of silicon 1/4-inch square holds the same capacity as the ENIAC, which occupied a full city block.