The Death Star's main purpose is to function as a mobile platform for its main weapon - the Superlaser. The Death Star's structure is basically an enormous housing for the Superlaser and the reactor that powers it.
It takes a lot of technicians to operate and maintain the Superlaser. And although it's the most powerful weapon in the galaxy, it's completely defenseless if attacked directly. So the Superlaser also needs military support to defend it. To address these issues, the designers of the Death Star equipped this enormous housing to serve two purposes: It is both a mobile weapons platform and a fully operational battle station.
Of course in order for the Death Star to be a real threat, it has to be mobile. To accomplish this the Death Star features a complex network of real-space ion engines and hyperdrive field generators that allow it to travel like any other interstellar space craft.
So basically the Death Star is made of four major components: the battle station, the Superlaser, the propulsion system and the hypermatter reactor that powers it all. Let's look at all of these components.