The Milky Way was the home of the humans- and their greatest enemy, the Ants. It was filled with millions of star systems, many colonized by humans. It's near destruction by the Ants caused it to be abandoned by the humans for the nearby Andromeda Galaxy.
The Core worlds was the name of the solar system and the surrounding systems. It contains Earth, the original home of humanity, where the government was located. Earth was the most populated planet in the galaxy and the richest. Most of the elite and high ranking officials lived here, causing class problems with the Outer Territories. The second most important core world was Mars, where the United Human Defense League was located. Mars was where most weapons and warships were created and where the highest officials in the military devised strategies. It was the site of the final battle of The Extermination War. The asteroid belt was the home of few, except for the asteroid miners, hoping to get rich of the precious metals within. The gas giants were the home of massive cities in the sky, and were common vacation spots for the rich. Mercury and Venus were far to hostile for human life.
The inner colonies were the majority of planets outside the core worlds. Having been established long before the Outer Territories, they were more civilized and developed. Many were mining or agriculture worlds, and provided the Core Worlds with needed supplies. Most of the inner colonies were destroyed during the war.
The Outer Territories were the frontier of the galaxy, where small colonies fought to survive. Life was hard and many were poor. This caused them to resent the Core Worlds. After The Extermination War started, the Outer Territories were cut off and spared the destruction of many inner colonies. Their perceived abandonment by the Core Worlds caused them to secede and form The Empire.
The dead planets were the worlds that had been sucked dry of their resources by the Ants. The dead planet zone spread into Humanities' territories as the Ants conquered more colonies. The Ant's destruction at the hands of Humanity stopped its advance, but too much of the galaxy had already been destroyed.