The Milky Way is the setting, and as such humans are at the forefront. The main race is humanity.
Centuries into the future mankind has invented the secret to faster-than-light travel. With it humanity has over many years populated the stars through a significant portion of the galaxy.
The first major breakthrough was that of subspace warp. A well-coordinated and delicate quantum equilibrium could be established, creating a quantum wavefront that changed time within it. Any object within the elliptical wavefront could travel at up to the speed of light. Although fuel couldn't be brought along, it was found that the floating hydrogen in space was enough. The first probes were sent out this way. However, the kinetic drive systems took a very long time to reach such speeds and as such this form of travel was inadequate for widespread use. The development of the relativistic drive allowed nearly instantaneous acceleration without destroying the craft and anything within it. One inside such a drive would experience little time. These twin inventions allowed humanity to travel at up to the speed of light, and within several decades colonization of distant star systems had begun. But it wasn't enough. Then came the gravitic drive. Utilizing the same concept of quantum wavefront, this new device shifted the gravity well around it, "sucking" crafts forward. It allowed magnitudes faster travel. The gravitic drive ushered in a new era of hyperspace warp travel of speeds of around 100,000 c (c being the speed of light), also known as warp-5. A whole swath of space underwent colonization. The gravitic drive takes 10 s. to go into and out of warp, potentially enough to win or lose a battle. With such powerful technologies, humanity would soon stretch across the entire galaxy, and come in contact with the many other species they would find on other planets. It also allowed very rapid transportation and communication (which is no faster than transportation). Trade routes were established across the stars. Gravitic drives are small and convenient to use. Generally, warpcraft are any vessel with warp travel capabilities, and warpships are frigate or larger vessels with such capabilities.
The ultimate in space travel would come with the creation of the first sustained wormholes, folding the very fabric of space-time to allow instantaneous travel, to be known as ultraspace warp. This is done with a tunnel drive. A single tunnel drive can open up a single, bi-directional wormhole very quickly, and that wormhole can allow an infinite number of ships to pass. The tunnel drive, however, required significant space and mass, around 2,400,000 cubic meters. Therefore only capital ships or larger vessels could utilize such travel. The wormhole takes 50 s. to establish and 25 s. to dissipate.
While in hyperspace units cannot attack, generally can't be attacked and can't do much sensing. It takes a short while to enter/exit hyperspace to/from negligible velocities. Battle can't happen when either side is warping. Warping units will destroy themselves if they try to strike out, and units will almost never hit units that are warping.
It is the year 2250. It has been 125 years since the discovery of the subspace warp. Significant human settlements could be found across a 45,000 light-year (ly) span of the Milky Way galaxy.
In the meantime various other technologies have also advanced. Ships have long been able to garner resources while en warp route, but now they are able to gather far more than they need. This means that the ships would be able to continue production while en route. Aided by rapid assembly construction, fleets could seemingly spring up overnight. Chemistry has advanced to the point of having a plastic or composite made from common-to-find-in-space materials for just about every imaginable purpose, eliminating the need for resources from many dozens of elements. The most common, hydrogen and helium, are the primary ingredients for many reactions. However, a few necessary elements, namely silicon, xenon, and titanium, are incredibly rare in space and must be brought along with the journey. This technology, chemicycling, has enabled certain warpships to become mobile factories.
With the shift toward military use of warpcraft, AI are being programmed to direct warpcraft for the first time in history. Computing capacity is almost inestimable, with planet-sized CPU's able to analyze just about anything down to its individual atoms, and with sensing equipment to match.
Although nanotechnology has been developed to a significant extent, much of human society feels revulsion against having them implanted or injected into the body, viewing it an ethical disgrace.
For a long time the unity and power embodied in the United Nations meant that all of humanity stood together. No true war had occurred in over 400 years, and almost no new weaponry developed. However, all was not well. The UN had already lost political significance over many of the fringe colonies, despite holding sway over all of the larger planets. As if there being a nearly infinite number of planets to settle and hold dominion over wasn't enough, millennia-old rivalries (namely the desire for hegemony) continue to get ever tenser. The United Nations, for centuries the bringer and keeper of peace for all of humanity, is weakening, more and more seen as obsolete. New alliances are forming across multitudes of planets. Military ships have been created on all sides and armed with modern-day weaponry and warp travel equipment, ready to spring to action the moment the UN crumbles. That day is coming ever closer. Earth, the UN's primal bastion, has a population equal to that of all other star systems' combined at 50 billion. The UN is not a closely-knit government the way it might appear. The hundreds of nations comprising it still are independent to a degree, and may withdraw support for the UN Security Council at any time (generally without fear of reprisal). Generally speaking, its structure has not changed much since its formation.
The second prominent organization after the UN is the Republic, which had peacefully split away from the UN after the latter acknowledged that having a single congressional body dominating all of colonized space was not going to work for long. Comprised of most of the colonized fringe worlds, it spans almost as much of the galaxy as does the UN. Therefore it is considered the most significant of the threats confronting the UN. However, as its economic, technological and military capacity is only 1/100 that of the UN's, and as most politicians in the Republic are closely allied with UN politicians the Republic is not truly much of a threat. The treaty of secession is now nearly a hundred years in the past, and due to good relationships currently has little risk of falling apart. Meanwhile, the UN rarely interferes in Republic matters. Contrary to its name, the Republic is a true democracy where everyone gets an equal vote on all policies, making it even more liberal than the UN (which is really a republic). The Republic has no political representatives, only administrative ones who are generally named Administrators and Bureaucrats.
The Alliance may be even smaller than the Republic, holding only a few dozen star systems, but due to the political situations of its treaty of secession from the UN its leadership has constantly doubted the desires of the UN for continuance of the Alliance as a separate political entity. As a result it has secretly developed a variety of projects for improving its capability to wage war should it ever become necessary. Its military, though mostly kept a secret from everyone else, is larger than that of the Republic and more technologically advanced (militarily) than the UN. Though it had no plans for launching a preemptive attack on either UN or Republic, recently its power hierarchy has become increasingly unstable, no doubt related to its continued focus on and resource drain toward the military. The Alliance's government is federated in structure, with each solar system its own state and having its own voice in both the Alliance overall government and (especially) authority over its own territory.
The Freelancers is a small but galactic-scale organization of criminals and political dissidents formed by necessity to stay out of trouble from any of the human governments. It has the smallest population and military of all the independent governments, but the highest ratio of warpships to people. Due to the hardships of having to keep out of harm's way they have developed a flexible and efficient military and war-based lifestyle which has allowed it to win small battles against fleets out to hunt them down. Its hierarchy is mainly top-down with a single Executive at the top who acts as an amicable despot might.
Cybernetic organisms, Cyborgs for short, are the creations of a particularly extensive, long-going and costly Alliance project. The result is effectively a new race - Cyborgs - that are basically humans with extensive modifications at the nanotechnological level.
Along with contemporary AI, the Cyborgs are effectively the ultimate in human military forces. Each Cyborg is connected to all others via the Cybernetic Nexus, allowing them to team together to accomplish their objectives with far greater efficiencies than possible to humans. The Nexus also means that they can share each others' thoughts and memories. However, as transportation speed is not infinite there is a delay between Cyborgs separated by significant distances. Since Cyborgs are still humans, they feel the compassion and emotions that humans do and thus aren't simply stop-at-nothing, unthinking antagonists. However, having seen what a Cybernetic Nexus has to offer humanity, they want to improve the well-being of the rest of society by incorporating them all into the Cybernetic Nexus (assimilating them), peacefully if possible. Peace, however, isn't present in their mission as the rest of humanity quickly (and wrongly) comes to see the Cyborgs as a nightmare race and strive to fight them.