Sweaters, Sweat Shirts, Jackets and Coat

Jacket: a short coat, usually opening down the front.
Sweaters: a knitted jacket or jersey, with or without sleeves.
Sweat Shirts: a loose, long-sleeved, with close-fitting or elastic cuffs and sometimes a drawstring at the waist, commonly worn during athletic activity for warmth or to induce sweating.
Coat: an outer garment with sleeves, covering at least the upper part of the body: a new fur coat.

Now you know the definitions. So, now when our friend calls there jacket a sweater you can feel smart and correct them. (Don't be a jerk about though) Then if you come to an argument you can just come here. If there still pretty sure that they are correct, you can go to http://dictionary.reference.com and get longer more complicated definitions.