first of all the shawl is a simple rectangular or square piece of cloth, you just put it on you with no buttons or clips. it is very loss and comparable , but if you do not fasten it strong enough it will fall.

A cape is a sophisticated shawl ,there are buttons ( or a clip ) and it fastening about the neck, with the opening on the front. thus it covers only the back half of the wearer. it can not fall and it is easy to use the ends while wearing it.

A poncho is a type of a cape, it is not open on the front and you "slip" in to it, it is more warmer then a cape but you can not manipulate your hands as well.