Both are open and airy garden structures that do not have solid roof structures, instead having a series of crossbeams often forming a grid pattern. They differ mostly in size and in there use. Pergolas are meant to offer shade and tend to shield a larger area. This may be freestanding or attached and can as a walkway or simply as a dedicated area for relaxing/entertaining. An Arbor is more of a smaller decorative feature. They may be arched (but not necessarily) and are often used to mark an entrance to a section of the garden/yard. They may also be fitted above or include a bench and used in scenic areas. Both forms are commonly used to grow vines or other climbing plants that can grow over the structure with Arbors often having a latticework up the sides (as well as the top) where a Pergola would only have posts/pillars.