Filo dough (also spelled as phyllo dough) is a thin type of pastry dough that differs from puff pastry. Filo dough originated in the Mediterranean region and eventually spread to other regions in the Eurasian subcontinent. Bakers include flour, water, fat, salt, and vinegar together to form very, very thin sheets of dough. Once the sheets are formed, the dough can be stacked, shaped, and formed into baked goods like sticky baklava and spanakopita.
Filo dough is very fragile, so once it is formed into sheets, the dough is frozen until it is ready to be used. If you mistakenly use filo before it is thawed, however, the dough will crack, split, and become useless. The dough also tends to dry very quickly because it is thin, so if you work with filo dough, you should complete all other preparation steps before removing the dough from your refrigerator.
If you plan on working with filo at home, you should know to keep melted butter handy. Traditionally, sheets of filo are stacked with butter in between and baked until golden brown and flaky. The result is a texture similar to puff pastry, but rarely will bakers substitute filo dough and puff pastry interchangeably. Puff pastry is much thicker and will not offer the same mouth-feel as thin filo pastry.