Short answers, pulled from the story.
The Spelljammer is a living biological ship shaped like a manta ray that consumes heat and light to produce air and food for its inhabitants. It serves as the namesake for the entire Spelljammer setting and can only exist as one true vessel at any given time.
TSR released the Spelljammer boxed set in 1989, which introduced Realmspace for the Forgotten Realms, Krynnspace for Dragonlance, and Greyspace for Greyhawk. The product line expanded with boxed sets and accessories such as Lost Ships in 1990, Realmspace in 1991, and The Astromundi Cluster in 1993.
Crystal spheres are separated by the phlogiston, a bright extremely combustible gas-like medium that exists between the spheres and flows in currents known as Flow rivers to facilitate travel. Portals in the sphere wall can open and close spontaneously, and ships or creatures passing through a portal when it closes may be cut in two.
Wizards of the Coast released a new boxed set titled Spelljammer: Adventures in Space on the 16th of August 2022, which updates the Spelljammer setting for the 5th Edition. This release includes a Dungeon Master's screen, a double-sided poster map and three 64-page hardcover books: Astral Adventurer's Guide, Boo's Astral Menagerie, and Light of Xaryxis.
The Spelljammer universe is populated by a diverse array of alien races including xenophobic beholders, rapacious neogi, militant giff, centaurlike dracons, hubristic elf armadas, spacefaring orcs called scro, mysterious arcane, the Thri-kreen insectoids, and bumbling tinker gnomes. Illithids were another major race, but were presented as more mercantile and less overtly evil than in other D&D settings.
Sixteen comics set in the Spelljammer universe were published by DC Comics between September 1990 and November 1991, with the creative team of Barbara Kesel as writer and Michael Collins, Dan Panosian, and other contributors. Six novels set in the Spelljammer universe were published by TSR before TSR was incorporated into Wizards of the Coast, and the novels were interconnected and formed The Cloakmaster Cycle.