Well, what I usually think of as a jacket is "marynarka" or "żakiet" (for women). Athough certainly different from "płaszcz", and often called a jacket, "kurtka" can sometimes also be referred to as a coat (especially the outdoor type). The PWN/Oxford "Wielki" lists both.
"płaszcz" as in overcoat or raincoat is "coat", yes, but there are other types of coat, some of which (like the one I wear) would be translated as "kurtka". Languages don't always have a 1:1 exact match in meaning.