Phyllo, Fillo, and Filo are just variations in the spelling of tissue-thin sheets of dough. The word means "leaf" in Greek as the sheets are as thin as a leaf. They're also quite delicate and become brittle if left exposed to the air. For that reason, a damp towel is placed over filo when it's out to be used in a recipe. Puff pastry, or pâte feuilletée in French, is a laminated dough, formed when chilled layers of butter are formed between layers of dough by folding, chilling, and refolding. Due to those layers of butter, the dough "puffs" when baked. Both filo and puff pastry can be found in the frozen section of many grocery stores. They are generally not interchangeable in recipes.