A bay leaf is a fragrant leaf from a laurel tree that is used as an herb. Bay leaves can be used fresh or dry, but dried ones tend to have a stronger flavor.
Fresh bay leaves are a shiny dark green on their tops with a duller, lighter green underside. When they're dried, they look pretty much the same on both sides.
Bay leaf is sometimes ground into a powder and used almost like a spice.
In addition to simmering them in soups and stews, bay leaves are great for stuffing into the cavity of a chicken before roasting it, and they can be added to the liquid for cooking rice.
There's a misconception that bay leaves are poisonous, but be assured that the practice of cooking with poisonous ingredients is very much frowned upon in the culinary arts. There do happen to be a couple of species of bay leaf that are poisonous, specifically the cherry laurel and mountain laurel, but these varieties aren't sold as herbs.
The reason bay leaves are removed before serving has to do with the fact that the leaves have sharp points and can stab you in the mouth if you bite down on them the wrong way.