Card sorting
Also called card sort, open card sort, closed card sort, hybrid card sort
Asking people to put a set of cards (pages, products, topics) into groups, to learn how they expect things to be organised.
In a closed sort the groups are yours, and the question is whether people put each card where you would. In an open sort people make and name the groups themselves, which shows the categories and the words they would use. A hybrid sort gives them your groups and lets them add their own when nothing fits.
The result to read is the placement matrix: for every card, the share of people who put it in each group. A card that lands in one group almost every time is settled. A card split between two groups is one your labels leave ambiguous, which is exactly where people will look in the wrong place on the real site.
Thirty to sixty cards is a comfortable sort; past that people tire and start dropping cards anywhere. Write the cards in the words people use, not in internal jargon, or you test the jargon.
Where it goes wrong
Reading open-sort group names literally. “Money”, “Billing” and “Payments” may be the same group in three people's words; merge them by meaning before counting.