Yes, I know, burning question right?! Well at Friendsgiving this question actually came up. There was a discussion whether you wore a jacket or a coat. At first we were going, "Oh, maybe it's the classic 'soda vs pop' and it just depends on where you're from". well, NOPE!
According to several sources, however I chose landsend.com to be my expert on this one, the difference is in the length! Jackets are shorter, usually coming to your waist or slightly below the waist. Coats are longer, usually ending at your mid-thigh and below. Coats are also usually heavier weight.