Card sorting is a user research technique used to understand how people categorize and understand information. It's primarily used by UX designers and information architects to create an intuitive and easy-to-use website or application.

In a card sorting session, participants are given a set of topics written on cards. They are then asked to sort these cards into categories that make sense to them. This could be done physically with actual cards, or digitally using online tools.

There are two main types of card sorting: open and closed. In open card sorting, participants create and name their own categories. In closed card sorting, participants sort cards into predefined categories.

Card sorting is used to understand the mental models of users, meaning how they structure a solution to a problem in their minds. This method provides insights on how users categorize information, which can help in designing a website or application that resonates with users' expectations and preferences.

Card sorting can also be conducted either in-person or online. While in-person card sorting allows for deeper insights as the researcher can probe respondents on their sorting decisions, online card sorting is beneficial for projects further along in the design process and can be more convenient for both the researcher and participants.

One of the key benefits of card sorting is that it helps you empathize with users. It provides a glimpse into the perspective of your user, allowing you to learn how they understand different concepts or ideas, and how they feel about them.

However, card sorting should not be used as the only research method. It is most effective when used in tandem with other user-centered design techniques such as usability testing and contextual inquiry.

To conduct a card sorting session, start by writing each item on an index card, Post-It note, or your own handmade card. Ask participants to arrange the cards into groups that are logical to them. Observe the user closely and occasionally inquire about their rationale. After the session, analyze similarities and differences between each person’s card sort.

In conclusion, card sorting is an effective and powerful tool in the design process. It provides valuable insights into the mental models of users, which can be used to create a more intuitive and user-friendly website or application.