Cognitive science is an interdisciplinary field devoted to exploring the nature of cognitive processes such as perception, reasoning, memory, attention, language, imagery, motor control, and problem-solving. The goal of cognitive science is to understand (1) the representations and processes in our minds that underwrite these capacities, (2) how they are acquired, and how they develop, and (3) how they are implemented in underlying hardware (biological or otherwise). Stated more simply, the goal of cognitive science is to understand how the mind works! Trying to understand our own minds is one of the most ambitious and exciting projects in all of science, and requires insights drawn from many different traditional academic disciplines. As a result, cognitive science is an inherently interdisciplinary endeavor, drawing on tools and ideas from fields including psychology, computer science, linguistics, philosophy, and neuroscience (among others).
Another description of this discipline can be found in the first few paragraphs of the bluebook description of the cognitive science major, and you can also check out the main Cognitive Science program website.