A bow is a type of knot generally featuring two loops and two loose ends. Bows will be seen on clothing (many serafuku have them), in hair, on panties and other lingerie, on shoes (shoelaces are usually tied in a bow), and pretty much any place that there is something that might be tied. Bows are often made of ribbons. If the material that makes up the tail of a bow is far longer then the width of the bow it should be noted as a ribbon.
Other senses of bow
source: danbooru wiki