Using her abilities as Zeus' daughter, Thalia can channel lightning through the spear, striking whomever she aims it at. It can also be used as a regular spear and was often her weapon of choice along with her shield, a replica of Aegis, that was made after her father's shield, forming a very formidable duo. However, it was replaced when she joined the Hunters, as her primary weapons then became hunting knives and a bow with a quiver of arrows, which both appear slung over her back when she needs them (as they are enchanted). She kept her spear, as well as Aegis, though the spear perhaps may only be used if she runs out of arrows.
In The Titan's Curse, Thalia says that it was given to her by Athena, but in The Demigod Diaries, it is found out that she took a replica of the shield Aegis, from Halcyon Green's mansion. The spear had been with her beforehand.