The Death Star which unlike its name is not a star but is a giant artificial planet built and controlled by the Galactic Empire that can be flown like any spaceship. The first Death Star was blown up by Luke Skywalker at the Battle of Yavin IV in Star Wars IV - A New Hope. The giant crater at the top of the planet is a weapon that can be used to destroy planets.
Grand Moff Tarkin is the person in charge of the first Death Star. Director Krennic had been the person in charge overseeing the construction of the place. Tarkin took charge claiming there had been a number of security breaches which had meant that Krennic was unfit to oversee the station. When the Empire hold Princess Leia prisoner onboard the Death Star, Tarkin orders the destruction of the planet of Alderaan in revenge for the Princess not revealing the location of the Rebellion`s hide out. Princess Leia had hidden the plans in the memory bank of R2-D2 who along with C-3PO had escaped capture when their craft Tantive IV was captured by Darth Vader..
When the first Death Star is destroyed, the Empire start about building a new one under the control of Moff Jerjerrod but it never gets finished as you can see from the picture above. Although not finished, the new Death Star is operational and is able to destroy one of the Rebel`s space craft that has arrived in the vicinity to attack it. The Death Star is protected by a shield that is generated from the nearby Moon of Endor.
A small team of rebels lead by General Han Solo are despatched to the planet to destroy the generator. Once the generator is down, a team led by Lando Calrissian flying the Millenium Falcon can lead an assault and destroy the second Death Star.
A plan to urge the U.S. Government to build a Death Star was kicked into the long grass when the U.S. Government said it was too expensive and they don`t support the blowing up of planets. 1
The diameter of the first Death Star was 120Km. To put this in to context and compare with real a real planet or moon, it is just a litte smaller than the Uranian moon Portia which has a radius of about 67km ( or diameter of 134km. The second Death Star was slightly bigger at 160km, that is fractionally smaller than the diameter of also another Uranian moon called Puck. Also the Saturnian moon of Mimas has the unenviable title being nicknamed after the Death Star because of the large crater it has and being also spherical.
In Star Wars - The Force Awakens, the Empire has now become the First Order and instead of building a new artificial planet, they`ve hollowed out an ice world and turned it into a new weapon. The new weapon is more powerful than the previous Death Stars as the new weapon can destroy whole star systems such as our own the solar system.
Star Wars - Rogue One is the first spin off of the film, it covers when the Death Star became operational and the effort by the small band of Rebel Alliance fighters who steal the plans for the Death Star. The spacestation weaponry was made possible by the involvement of Galen Erso, the father of the lead protagonist of the film, Jyn Erso. Unbeknown to the Empire, Galen had installed a fault in the Death Star which would allow a single pilot to blow up the station with a carefully placed shot. The plans for the Death Star were in the research station on Scarif which is where Jyn, Cassian and a group of other Rebel Alliance soldiers recovered the plans for.
The first planet that the Death Star destroyed was Alderaan, home of Princess Leia. Before Alderaan, all the station had been used against were cities of Jedha capital and the research base on Scarif which had to be destroyed to prevent the plans from getting off the planet. The plans are transmitted to the Rebel Alliance fleet in orbit which are then passed onto the Tantive Ive.
The main weapon on the Death Star is powered by Kyber Crystals, the same that powers the lightsabres of Jedi and Sith lords. The Empire has mined Jedha for Crystals and refines them on the rocky wet planet of Eadu which is also a site of a small battle between the Empire and the Rebel Alliance.