Lima Prime
The most heavily populated planet in the sector, the Hive World of Lima Prime is also the sector capital. As far as Hive Worlds go, Lima Prime would make a good archetype: an overpopulated, heavily polluted planet featuring massive hive cities with vast stretches of wasteland between them. The only acceptable living conditions can be found in the upper echelons of the hives, where the planet's ruling class make their home.
Lima Prime is plagued by the usual troubles of Hive Worlds: the hives support large criminal networks, coupled with an abundance of cults of various sorts. The local Arbites have their hands full suppressing these cults, which thus far they don't sport much influence on the populace.
Lima Secundus
The second planet of the Lima system is a dead world with an atomsphere, a rather unusual combination. While life could exist on Lima Secundus, it simply doesn't, for unknown reasons. Most of the world is covered by sand, giant stretches of deserts only interrupted by mountains and rock formations.
Why the Orks came to Lima Secundus, nobody knows for certain. The most likely theory involves a crashed Space Hulk populated with greenskins, but thus far no crashsite was found (but keep in mind that all observations have to be done from a safe distance), nor was there any Imperial witness to said crash. As the dominant Ork clan on the planet is the Evil Sunz, it's speculated that the hulk was completely dismantled and transformed into vehicles of all sizes.
As far as the Imperial scholars know, the Orks fought a great many skirmishes and battles until the Evil Sunz warband named Grin Skorchers emerged victorious, beating all other clans and warbands to the brink of extinction.
In 988.M41 the Planetary Defense Force of Lima Prime launched an preemtive strike against the Orks of Lima Secundus, but this attack was a catastrophic failure, only rallying the Orks under the banner of the Grin Skorchers that much quicker and heavily depleting the PDF.