Here are the four most common research methods:
Usability testing. This method evaluates how easy it is to use an interface or product. These tests can be done on a prototype of any fidelity and any live site or piece of software, including competitors. These tests are useful to help you find out if there are issues with your product that impede a positive experience. Usability tests are usually moderated sessions, whether remote or in person, where user sessions are recorded and studied.
User interviews. This method is used to get qualitative information from users. It's basically an interview where you ask the user questions about their needs, motivations, and goals. An ethnographic interview is one of the best ways to do this. A user interview can help you and your team define user personas.
Card sorting. This method helps you categorize information and determine hierarchy for information architecture. The two variants to this method are called open card sorting and closed card sorting. Open sort is where the participants organize the cards in a way that it makes sense to them. Closed sort requires you to give the participants defined categories and they have to move items wherever they best fit.
Tree testing. This method is linked to card sorting. Once you have your categories defined, you provide your tree structure to a user and ask them to find an item using that navigational structure.