The Droids distinguish themselves from most other species, because they are no humanoid species of flesh and blood. They are completely mechanical, yet undependent from any other species. They have very highly developped Artificial Intelligence routines, that they are capable of expanding. So in a way, they are able to evolve on their own.
To understand better who the Droids are, and where they come from, we must look back to the history of the Traquel species, a humanoid race in the Delta Quadrant that lived rather close to their neighbours, the Borg. Traquel are a highly developped species with alot of technical skills. The Borg was soon interested in assimilating their knowledge, and attacked the Traquel homeworld, with the loss of many of their people who were assimilated that day. The Traquel thereafter decided to develop a form of deefense against future Borg invasions. This form of defense was to be the Droids.
Droids cannot be assimilated, because they are no lifeform of flesh and blood. Even so, the Borg would be interested in assimilating the technology, but this they have found rather hard to do. The Traquel was able to create their Droids with such intelligence and precision, that the technology rejects any other form of technology they might get linked to. Result: when a Droid is captured by the Borg and getts assimilated, its technology will not be compatible with any kind of Borg technology, and both systems will reject one another, whereafter the Borg will believe this technology to be unworthy of their time, and they will lose interest in assimilating this technology any further. The Droids were meant to be the guards of the Traquel, and hold back any Borg attack there might be. Even so, the Droids were programmed with such advanced A.I. routines, that they soon evolved to a state where they wished to live on their own, independent from the Traquel. The Traquel, understanding how this came to pass, uaccepted this and allowed the Droids a part of their own space. The Droids formed their own civilisation, called the "Droid Nexus" and expanded into deeper space. They now occupy three planets, called Unit 01, Unit 02 and Unit 03. Even though the droids are no longer depending on the Traquel, they did not forget their primary mission: to defend the Traquel from the Borg. As Droid space is even closer located to Borg space, they form the perfect wall of defense.
The Droid society is a very organised one. Their first planet where they settled, once located in Traquel space, and now part of Droid Nexus space, is still relatively close to Traquel space, while the second is already further away. The third planet is located in System J25, a sector of space partially inhabited by the Borg as well. Unit 03, the name of the planet located in J25, is very strategically chosen to settle on. This planet's location lies within some nebula, which makes it nearly impossible for the Borg to mount surprise attacks.
The Droids did not choose Class M planets to settle on, but rather Class D planets, usually large asteroid planets or moons with nothing but a rocky and barren surface, without any oxygen atmosphere. After all, Droids are not humanoids and thus they do not need oxygren to breath and survive. Droid planets generally have a grey look on the outside, since Droids are very productive units who continue building on their cities until those cities have completely covered their planet. In fact, a Droid planet is one large city and it takes a digital mind indeed to find your way back there.