Explanation of Pensieve

Dumbledore: "I use the Pensieve. One simply siphons the excess thoughts from one's mind, pours them into the basin, and examines them at one's leisure. It becomes easier to spot patterns and links, you understand, when they are in this form."
Harry: "You mean... that stuff's your thoughts?"
Dumbledore: "Certainly."

- Albus Dumbledore to Harry Potter

• The Pensieve is an object used to review memories.
• It has the appearance of a shallow stone basin, into which are carved runes and strange symbols. It is filled with a silvery substance that appears to be a cloud-like liquid/gas; the collected memories of people who have siphoned their recollections into it.
• Memories can then be viewed from a third-person point of view.

Etymology

• Pensieve seems to be a pun, combining the words "pensive" and "sieve".
• The latter is an object in which something may be sorted, thus it allows for the sorting of thoughts, or memories.
• It is also an anagram of Pevensie, the surname of the characters Peter, Susan, Edmund, and Lucy from The Chronicles of Narnia whom the author (C.S. Lewis) of was J.K. Rowling's favourite.

[Adapted from http://harrypotter.wikia.com/wiki/Pensieve]