Card sorts are a quick and dirty way to organize ideas, thoughts or concepts into themes. Cards can be pre-populated with aspects of your product design or you can ask users to populate the cards with features and functions they come up with themselves that are related to your problem/solution. Users are then asked to organize the cards in the way that seems most natural to them.
Card sorts are used to organize information ie: identifying natural groups, understanding user intuition, informing navigation etc
Card sorts open your eyes to the ways that others see the words, topics and ideas that are relevant to your venture. Sorts provide insights into user expectations regarding both your organization system and the priorities related to specific functions.
Cards and sticky tack or Post its, sharpies, table and wall space OR online card sort tool.