Cognitive Science is the endeavour to model thought processes and cognitive representations (and the subsequent intelligent behaviour that arise from them) in purely scientific terms. Currently, the main focus of Cog. Sci. is to resolve the behavioural and cognitive levels of mental activity - the biological aspect is not given as much attention, because we currently lack the tools to investigate it thoroughly. Cog. Sci. has very strong links with Linguistics and Artificial Intelligence (Chomsky has a lot to say about the subject). It's currently quite a young discipline, with the potential to branch off in several directions and to alter its objectives and stance on several key issues as advances in its related fields filter through.

If the human brain was simple enough for us to understand, we'd be too simple to understand it.