Definition
A pergola is a structure built with columns or posts that support a series of rafters or latticework that serves as a cover for a patio or a promenade (a walkway) that typically connects the garden to the home. The pergola does not have a finished roof and often is used to support vines and plants that provide shade and shelter. In the photo you can see an example of how the rafters are placed and rest on the pergola frame.
Construction
A pergola is constructed using posts or columns that are attached to concrete footing that extends below the frost line. The frame of the pergola can take many forms in order to provide a secure structure, but the rafters (typically built using wooden beams that are plain or decorated) run across the frame spaced about 24-30 inches apart. This provides gaps that are not too far apart for vine growth. If the pergola is built close to the home, the rafters may run from the roof of the home to posts and columns on the outside as shown by the pergola outside the monastery in Crete.