Card sorting is a cheap, and relatively fast technique to help with categorization. Although not directly relevant to the user interface, it can have a significant impact on your user interface if your categories are poorly defined.
Making things as easy as possible for your users to navigate your website or your software, for that matter, is of utmost importance.
Card sorting is one technique that can help a lot with this. I found this excellent write-up for this.
<a title="Boxes and Arrows - Card sorting article" href="http://www.boxesandarrows.com/view/card_sorting_a_definitive_guide">http://www.boxesandarrows.com/view/card_sorting_a_definitive_guide</a>
I disagree that card sorting does not have a direct impact on the user interface. Card sorting is not just about categorizing content; it is also a tool for helping to discover user's mental models about information and discovering the terminology users employ when browsing labels and doing searching. The interface is all of the text on the screen, the metaphors employed to convey meaning, and the interaction model. What you call a button, for example, should tell the user what to expect when the click the button. Card sorting or other forms of sorting help you decide what word is most recogniable and meaningful to users.