Crackers were named for the acronym "CRC" which stands for Climatological Reconfiguring Catalyst. In the simplest possible terms, a Cracker converts the atmosphere of a planet it hits from breathable to unbreathable in less than an hour via a process of high-speed deterraforming. This results, somewhat obviously, in the death of the vast majority of people on the planet. Only those with quick access to a space ship or some other form of artificial environment had any hope of surviving.
Crackers were popular because they left the resources of a planet largely intact, and open to whoever got to them first. However, this often lead to battles around dead planets between groups trying to claim rights of salvage, to say nothing of the ships in that system, often crewed by people who had friends and family on the "cracked" planet, seeking revenge on the scavengers. In response to the use of Crackers, full planetary shielding, such as Gaurron shielding, was quickly developed, since most techniques of surveillance were insufficient for detecting something the size of a Cracker, as opposed to the size of a spaceship or asteroid.
The Union quickly attempted to ban the use of Crackers. However, as many of the groups involved in the Orbital Wars no longer recognized their authority, this didn't help matters much. Indeed, many saw it as an incredible act of hubris to declare the use of Crackers illegal almost immediately after the Antiphon was used, and a few of these groups decided that the Earth should get a taste of its own medicine. During the fourth phase of the Orbital Wars, no less than three Crackers struck the planet that some claimed was the origin of all human life. Needless to say, all but the first were largely redundant.
The Union, which at the time was heavily based on Earth, although it purported to rule the entire Galaxy, was struck a blow from which it never fully recovered. However, from the metaphorical ashes of Earth's civilizations arose the [New Unified Terrans]?, whose role in the rest of the Orbital Wars was quite possibly more important (although much more warlike) than the role of the Union in the first three phases. Also, it should be noted that during the final years of the Orbital Wars, several groups and companies, primarily lead by Pharmacon?, began the reterraforming of Earth. This symbol of reconstruction and rebirth wasn't lost on many of the sides eager for a reason to seek peace.