The Delta is a expendable medium capacity launch vehicle first flown in 1960. The [Douglas Aircraft Company] was commisioned to build the Delta in April of 1959, as a civilian launch vehicle based on the [Thor] [Intermediate Range Ballistic Missile]. It was originally called the Thor-Delta, since it was the fourth modification of the Thor missile.

The first Delta launch failed due to an attitude control problem in the second stage, but the next 22 launches were successful. The original Delta [spacecraft] could place up to 100 pounds in [geostationary orbit] or 500 pounds into low earth orbit, but in 1962 Douglas Aircraft started a series of modifications and enhancements that would increase Delta's capacity tenfold over the next nine years. Since then dozens of additional [configurations] have been flown.

The most powerful current configuration is the Delta III, which is capable of boosting 18,280 pounds to [low earth orbit] or 6,000 to geosynchronous orbit. The Delta III is about 130 feet tall and uses a [Rocketdyne] RS-27A motor in its first stage, which burns liquid oxygen and kerosene to produce 244,100 pounds of thrust, assisted by nine Alliant [solid-fuel boosters] (six of which are ignited at lift-off and three are ignited in-flight) that produce 1,244,100 pounds of thrust. The second stage uses a Pratt & Whitney RL10B-2 engine, which burns liquid oxygen and hydrogen, which has a push of 24,750 pounds and can be turned off and re-started.

The Deltas have been the workhorse of [satellite] deployment. Many telecommunications satellites, most weather satellites, all of the [Global Positioning System] satellites and a variety of scientific and explorational satellites and probes rode into service atop Deltas. As of this writing, Delta has seen over 40 years of service, with 292 flights, only 15 of which were total failures (a success rate of over 94 percent).

A Delta IV is being developed by [Boeing], but it's a complete redesign with almost no similarities to the existing Delta line.

Sources:
http://kevin.forsyth.net/delta/
http://pao.gsfc.nasa.gov/gsfc/lithos/delta/delta.htm

delta (thing)