In No Man's Sky, galaxies are made up of smaller regions, as can be seen in the Galactic Map view. In some of the videos, so far, Sean has made it sound like certain groups of NPCs will mainly be found in these regions. Using the Korvax race that has been used in the pre-release demos as an example, the region that they call home would be the only region that contains their monoliths, and the architecture and ships would show their influence. So moving from one region to another, a player will see more of another NPC, and would see the influence in the respective architecture, ships, and language.
Where the regions border each other, Sean has made it sound like the different alien races can be at war and will battle each other in space.