They are dried orange peels that you can get them from your home brew supplier. Their names say it all bitter adds a bitter flavor sweet adds a sweet flavor. Personally I think both do more for aroma than they do for flavor. You would generally use them with the flavor hop addition around 15 min until end of boil. If you don't want to spend the money on the bitter or sweet orange you can use orange zest from store bought oranges. I am no expert but I was warned to stay away from the white pith when using store bough oranges. That being said I know some that throw the whole orange into the boil after squeezing the juice in.
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I just wanted to have a signature!