> What's Cognitive Science and what does one do in that field?

Cognitive Science is a multidisciplinary wossname combining psychology, philosophy, linguistics, computer science, neuroanatomy, and just about everything else that has anything to do with how and why people think.
It's primarily a research thingy; the idea is that a person with a background in more than one of these subjects might come up with something that somebody who'd only studied one of them would miss.
What one actually does in the field depends on what one studied. In my case, I focussed on Comp. Sci. and Psychology, so I'm potentially a useful Artificial Intelligence researcher. Had I chosen to focus, within Psychology, on educational psychology, I might have been facing a future in creating programs for computer-assisted learning. Somebody who focussed on Linguistics, say, instead of Comp. Sci., would be up to something else entirely.

In my case, I basically chose the course because I wasn't sure whether I wanted to study Comp. Sci. or Psych. more, and it allowed me to do both.

Paul