• What is a pensieve?
A Pensieve is a stone receptacle in which to store memories. Covered in mystic runes, it has liquid or gas within its basin. A witch or wizard can extract their own memories or someone else's and store them in the Pensieve and review them later. It also relieves the mind when it becomes flooded with information. Anyone can examine the memories in the Pensieve, which also allows viewers to fully immerse themselves in the memories stored within, much like a magical form of virtual reality. Tom Riddle's diary seems to have this same form of virtual reality. Oddly, users of these devices view the memories from a third person view, providing a near-omniscient perspective of the events preserved. This of course, raises questions of how they are able to see things beyond what they have remembered. Rowling answered this question in an interview, confirming that memories in the pensieve allow one to view details of things that happened even if they did not notice or remember them, and stated that "that's the magic of the Pensieve, what brings it alive". The contents of a Pensieve, as mentioned in the book, look neither like smoke nor like water. The "memory" has the appearance of silver threads. Memories that have been heavily manipulated or tampered with to alter perspectives (such as Horace Slughorn's) may appear thick and jelly-like and offer obscure clarity upon viewing. Memories are not limited to just those of humans, since at least one house-elf provided Dumbledore with one as well.
Like many names in these books, Pensieve is a pun: it is a sieve in that it is a device used for sifting out thoughts, and in using it one becomes pensive or thoughtful. It may be notable that "Pensieve" is an anagram of Pevensie, the surname of the main characters from C. S. Lewis's The Lion, the Witch and the Wardrobe, in which the Pevensie children are thrust into another world through a magical wardrobe, as Harry is thrust into a memory through the Pensieve in Dumbledore's cupboard.
A Pensieve first appears in Harry Potter and the Goblet of Fire, again in Harry Potter and the Order of the Phoenix, and plays a pivotal role in Harry Potter and the Half-Blood Prince. (Taken from Wikipedia)