Card sorting is a categorization method where users sort cards depicting various concepts into categories. You start with a list of all the items you want sorted. Write down each item on a separate index card. Give your user(s) the stack of cards and have them divide the cards up into piles, telling them that the cards should be grouped the way they (the users) best see fit. This technique is best used in the early stages of development. For more information see: Card Sorting.

a useful low fidelity discovery technique

A user centred design technique where you ask users to arrange your website content into groups that they would expect o see on your website. Typically involves groups or individual users rearranging cards & post-it notes on a table or wall.