A Pensieve is a wide and shallow stone or metal basin, intricately decorated with carvings or precious stones. The Hogwarts Pensieve, kept in the Headmaster's office, is described as a shallow stone basin with odd carvings around the rim, which appear to be a mixture of runes and symbols. The basin is filled with a mysterious, light, silvery-white substance that is neither liquid nor gas, resembling a swirling cloud or spun-silver hair. This substance is the physical manifestation of thoughts and memories. When a memory is added, it swirls within the basin, and the surface may clear to show the scene within. The memory itself, when extracted from a person's mind with a wand, looks like a silvery thread.
The primary function of a Pensieve is to store and review memories. A wizard or witch can extract a specific thought or memory from their mind and place it into the basin. This serves two purposes: it allows the owner to examine memories at their leisure and from an objective, third-person perspective, free from the emotional distortion that accompanies direct recollection. It also helps to clear one's mind when it feels cluttered with too many thoughts, a reason Albus Dumbledore cited for his own use of the device. To use the Pensieve, one can either dip their face into the silvery substance to be fully immersed in the memory, or simply peer into the basin to watch the scene unfold from above. When immersed, the viewer becomes an invisible, non-corporeal spectator within the recalled event. They are not bound by the original owner's point of view and can move around to observe details the owner may have missed or forgotten. However, they cannot interact with or alter the events of the memory. Multiple people can enter the same memory simultaneously.
The Pensieve located at Hogwarts is an ancient magical artefact that has been in the castle for centuries, passed down through successive Headmasters and Headmistresses. Albus Dumbledore made extensive use of it to store and analyze the vast number of memories he collected, particularly those pertaining to the life of Lord Voldemort. Severus Snape also owned a Pensieve. He used it during Harry Potter's Occlumency lessons to hide his personal memories, especially those involving Lily Evans, which he did not want Harry to see. It was this Pensieve that Harry fell into, witnessing “Snape's Worst Memory.” After Snape's death, he left Harry a collection of his most crucial memories. Harry used the Headmaster's Pensieve in the Headmaster's office to view them, finally learning the full truth of Snape's allegiance, Dumbledore's grand plan, and the fact that Harry himself was a Horcrux.
The Pensieve is a critical plot device used to deliver key exposition and backstory throughout the series.