The Death Star is a moon-sized starship, one of the largest in the Star Wars universe. Armed with a terrifying superlaser capable of destroying an entire planet, regardless of defenses, it was intended to be a symbol of 'control through fear' in the Tarkin Doctrine. The first Death Star was used by Grand Moff Tarkin to destroy Princess Leia's home planet of Alderaan.
Death Star Specifications
- Diameter: 160km
- Power Generation: Hypermatter reactor generating at least 1e33W
- Hyperdrive Speed: Not as fast as the Millenium Falcon (need real data stat!)
- Sublight Acceleration: >100Gs 
- Weapons: Main weapon yield approximately 1E38J 
- Recharge Time (Planet Scattering): less than 24 hours
- Other Abilities:
Death Star II Specifications
- Diameter: 900km
- Power Generation: Hypermatter reactor
- Hyperdrive Speed: Unknown
- Sublight Acceleration: Unknown
- Recharge Time (ISD-destroying): several minutes at maximum
- Recharge Time (Planet Scattering): Unknown
- Other Abilities: Off-Axis Firing 
The plans for the Death Star were first mentioned in Star Wars II: Attack of the Clones. Toward the end, as the Trade Federation leaders were fleeing Geonosis, Count Dooku was given the technical schematics which would later go on to become the Death Star I. These plans were taken by Count Dooku to Darth Sidious, the Sith lord. Full scale construction on the Death Star would not begin for many years, however. Practical application of so many new technologies required a great deal of research and development first.
Early development of the Death Star was slow and frequently hampered by ill-fated sabotage efforts. Many of those working on the project or aware of its existence doubted its completion. Effective research and development was not possible under the current work conditions. Moff Tarkin, in an effort to combat this fact, recruited or forced a small number of scientists to work on its development in a new installation set in the middle of a cluster of black holes, the Maw installation. It was here that the many breakthroughs necessary for the completion of the Death Star occurred, namely a fully operation superlaser.
After the completion and demonstration of a workable prototype of the Death Star in the Maw Cluster, Bevel Lemelisk, one of the main researchers and developers, took the plans to the Outer Rim world Despayre. The metal needed for the construction was mined from this world by slaves. Bevel Lemelisk then oversaw the construction of the first Death Star above Despayre. To mark its completion and test the superlaser, Despayre, the cradle world of the Death Star, was destroyed.
The Destruction of the First Death Star
Shortly after its completion and declassification, a rebel mercenary, Kyle Katarn, stole a set of the plans from the imperial data vault located on Denauta. These plans were transmitted to rebels on Toprawa. These plans were combined with similar data obtained by Keyan Farlander and other spies to produce a complete schematic of the Death Star.
After its use on Alderaan in an attempt to coerce the location of the Rebel base out of Princess Leia Organa, Darth Vader placed a tracking device on the Millenium Falcon, the ship used by Han Solo, Luke Skywalker, Chewbacca, and Obi-wan Kenobi to board the Death Star, rescue Princess Leia, and escape. The Death Star followed this tracking signal to the Yavin system, where the Rebel base was located on Yavin IV. Using the schematics constructed from the data obtained by Kyle Katarn and Keyan Farlander, rebel strategists had found a weakness in the battle station: a small auxiliary exhaust port located under the larger main exhaust port. If a proton torpedo was shot into this port, it could follow it in a direct line to the hypermatter reactor located in the core of the Death Star. This would set off what has been described vaguely as a "chain reaction", destroying the Death Star. Whether this phrase refers to a chain reaction located solely within the reactor or one throughout all the systems of the Death Star is unknown. Many believe it refers solely to the reactor due to the instantaneous destruction of the Death Star, but no consensus has been reached, and no statement has been made by George Lucas or any of his affiliated companies.
Small snubfighters were launched in squadrons in an attack at the Death Star. To reach the exhaust port they were required to advance down a trench along the equator of the Death Star, after destroying shield towers which prevented entry. The vast majority of the pilots sent on this mission were shot down by turbolaser towers, opposing fighters, or Darth Vader himself, piloting an Advanced TIE. Luke Skywalker was eventually able to successfully make the shot, and the Death Star was destroyed, killing Tarkin and many of the political and military elite of the Empire.
The Death Star's main superlaser is shown in to be capable of destroying an entire planet. This feat requires, at the absolute minimum, that the gravitational binding energy of the planet be overcome by the power of the weapon. That is, the weapon adds enough energy to the planet to scatter its mass quickly enough so that the planet's self-gravity is unable to pull it back together. The minimum energy for this task is within the range of 2.2e32 - 3.7e32 J, with 2.4e32 J being the most likely figure, as revealed on Mike Wong's Death Star Firepower page.
Observation of the destruction of Alderaan shows that the planet explodes violently, the main debris field expanding somewhere between 10,000 and 15,000 km/s, or 3-5%c. Assuming a simplistic distribution of velocities from 0 to ~4%c reveals this feat to require on the order of 1e38 J, or the amount of energy that the Sun produces in eight thousand years.
The Second Death Star
Immediately after the destruction of the Death Star, Palpatine demanded that the station be reconstructed. A station such as the Death Star was integral to the Tarkin Doctrine, in which fear rather than direct control rules the population. The Death Star II was not going to be a direct carbon copy of the original. Many design flaws that had been present in the first Death Star, including the small exhaust port that facilitated its destruction, were to be corrected. Also, Emperor Palpatine wanted a larger, more powerful station.
Bevel Lemelisk, the original Death Star's designer, also oversaw the design and construction of the Death Star II. Although he had been executed for the design flaws in the original Death Star, Palpatine cloned him and set him to task of redesigning the Death Star's original plans, correcting the flaws that had been in the Death Star.
As a result of this retooling, the Death Star II was a much different station, namely in size. The original Death Star was 160 km in diameters, the Death Star II's diameter was 900 km. This made the Death Star II 177 times larger in volume than the original. Also, the 2 m wide exhaust port the Rebels used to destroy the first station was now millions of millimeter wide holes spaced evenly across the entire surface of the station. This ports were also designed to close immediately if any excess energy was detected. Also, the turbolaser towers located on the surface of the station were increased in number and also given new targeting systems, making them more effective at targeting snubfighters on low-level flight paths, a duty they failed horribly at on the first Death Star. a further improvement, and probably the most important at the Battle of Yavin, was a complete redesigning of the superlaser. The first Death Star's superlaser was capable of destroying planets, but had a 24 hour recharge time, and no variable setting. The Death Star II was given a superlaser with much greater capabilities and flexibility. The superlaser was capable of off-axis firing, or firing at targets not directly lined up with the crater shaped firing area. Also, the superlaser was given variable settings and improved tracking and targeting systems, giving it the capability to fire at capital ships, an ability used judiciously at the Battle of Endor.
Since the majority main technical and development problems potentially encountered by the Empire in the construction of the second Death Star had been solved during the construction of the first Death Star, the Death Star II was built on a much quicker time scale, taking only two to three years to construct, despite its huge size and complexity.
The second Death Star's original construction site is not known; it was successfully kept a secret form Alliance spies. The Death star II was moved from its original construction area to the Endor system, located on the outer rim, because of the heavy concentration of metals and required resources on the worlds of that system. The Death Star II was put in geosynchronous orbit around the forested Sanctuary Moon, protected by a shield projected by a moon-based shield generator. All resources not obtainable in the local system were secretly shipped in using many carriers. One of these carriers was the personal transport company of Prince Xizor, who passed on the location of the second Death Star to the rebels.
Rebel Discovery of the Death Star II
Information used in locating the Death Star II came from many different sources, including imperial defectors and Prince Xizor. The majority of the information however, including the fact that the massive construction project was the construction of a second Death Star, came from a captured computer, stolen from an Imperial freighter. Bothan hackers decrypted the information and learned of the existence of the Death Star II, and also its location.
After learning of this information, Bothan spies were able to infiltrate the construction in the Endor system on three freighters. They confirmed the existence of the Death Star II, its shielding, and its state on progress. They also learned that its superlaser was not operational, a fact which proved to be false. The freighters were discovered and two destroyed, with the lone escaping freighter relaying the information to the Rebel Alliance.
The Rebels would go on to act on this information and destroy the Death Star II during The Battle of Endor.