Card sorts are generative research activities that ask participants to organize content based on their understanding of that content and how they think about it.
Card sorting activities can be as simple as getting a group of people in a room with sticky notes and pens, or as advanced as using online tools that allow people to participate remotely and researchers gather insights in real-time. Either way, card sorting is a great way to organize lots of information in a way that makes the most sense for users.
A Tree Test is a user research method that evaluates a site or tool's information architecture.
Information architecture is the way designers organize and structure information to make it easy for users to understand complex systems.