A pergola is a landscaping structure very similar to an arbor, but there are differences between the two. If we look at the subtleties, the following distinction can be drawn:
Garden arbors are simple, relatively small structures; often, they are arched at the top.
Pergolas are larger structures. Sometimes given greater architectural treatment, they may exhibit masonry columns, for instance.
Traditional pergola design harks back to grand masonry pergolas of the Italian Renaissance. But the term, "pergola" is used more loosely now, and includes wooden structures (as in the photo at right).
Like garden arbors and latticework, pergolas are often used as support structures for vines. The vines form a canopy over the pergola, affording shade in summer. But to afford complete shade, plus protection from rain, some people cover their pergolas. I have seen fiberglass used as a covering, but the more upscale homeowners may be interested in retractable shade canopies.
In answering the question, What is a pergola? some experts would distinguish pergolas from arbors by noting that the columns of the former form something of a "colonnade." For instance, Landscape Architect, Cynthia Cash writes, "The primary difference in an 'arbor' and a 'pergola' is that an arbor is a free-standing structure (also used to support vines), whereas a pergola is a long linear structure over a garden pathway."