Spelljammers are magic ships that fly through "wildspace" to various planets. All the planets and wildspace for a system are contained in a Crystal Sphere. Outside the Crystal Sphere is the Phlogiston, also known as the "flow". Holes in the Crystal Sphere allow the flow to shine through, appearing as stars on planets. Spelljamming ships cannot enter the Phlogiston.
In the Spelljammer game the player captains a ship and crew. The player can ship goods from planet to planet for a fee, take on simple missions including, delivering people and goods, destroying pirates, and guarding the space lanes. As the player completes missions his character gains reputation points, eventually gaining enough points to be asked to help rid Realmspace of a terrible danger. This mission led to the simple plot in the game.
The game starts the player with a 5th level character. The available classes are: Clerics, Fighters, Paladins, Rangers, Mages, and Thieves. There are three main parts to the game: docked at a planet, in space travel and ship-to-ship combat, and on-board personal combat. When at a planet, various text menus allowed the player to buy and sell cargo, repair and upgrade their ship, buy and sell character equipment, and seek missions. A rudimentary trade system allowed players to make money shipping goods from planet to planet, and some key planets had more options for ship repairs and equipment. In space, the player can travel from planet to planet, or simply point the ship in any direction and go. Real-time ship-to-ship combat using ballistae and catapults takes place in a simple 3D view. The player can steer his ship to aim the weapons at his enemy, and to try to avoid enemy missiles. The occasional meteor storm is a minor hazard while spelljamming.
One little-used feature programmed into the game was the ability to crash your ship into the Crystal Sphere. Normally "space" was shown as black, but if a player was patient and continued towards the Crystal sphere space would turn dark grey, light grey, the stars would spread out, and eventually the ship would crash into the Crystal Sphere.
The player can bring his ship close to the enemy ship, board, and do turn based hand-to-hand combat on a simple isometric map. A full range of spells and weapons is available, and ships are crewed by a wide range of humans and monsters. In the turn-based combat system, the player controls the ship's officers, and the computer controls the regular crew and the enemies.
Source: Wikipedia, Spelljammer: Pirates of Realmspace , available under the CC-BY-SA License.