In Swedish, the position of the article (en) determines its meaning. Thus, "en katt" means "a cat," while "katten" means "the cat."
It is a nifty feature of the Swedish language.
Think of it this way: "The Cat" is much more specific than just any "a cat." Thus, the article attaches itself to the noun in the former case, but not the latter. (katten vs en katt)