The primary hull (or saucer section or saucer module) is a major component of the overall hull structure of a starship. In many starship designs, the primary hull is usually located above and in front of the secondary hull of the vessel.
In the Federation, the primary hull is commonly referred to as the saucer section due to the rougly spherical shape that had been used in starship design since the NX-class starships of the 22nd century. This spherical design later carried into future starship classes such as the Constitution and Galaxy classes. (Star Trek: Enterprise, Star Trek: The Original Series and Star Trek: The Next Generation)
The primary hull of a starship often contains the most vital systems and facilities that are essential such as the bridge, computer cores, sickbay, science labs, and crew quarters. However, due to safety concerns, the main propulsion systems are usually housed in the secondary hull of a starship.
Since the 23rd century, Starfleet has often deployed starships that were capable of saucer separation. This capability would allow the primary hull to serve as a lifeboat that would allow the crew of a starship to escape a catastrophic disaster such as a warp core breach or loss of antimatter containment.
However, this capability was extended in the 24th century aboard some starship classes, such as the Galaxy-class, which would allow civilians and non-essential personnel to escape in the primary hull, while the secondary hull, which was equipped with the main armaments, would go into battle. (TNG episode: "Encounter at Farpoint"; movie: Generations; et al.)
The ability was extended even further towards the end of the century with the introduction of the Prometheus class which could routinely separate in a tactic called multi-vector assault mode where a single ships breaks into three parts to attack from multiple directions. (VOY episode: "Message in a Bottle")