As a mostly aesthetic product --- unless you go for an arbor with productive accessories! --- a large part of what you're buying when you purchase an arbor is its design capabilities. You'll find a large array of differing patterns and designs on the market, including:
Arched Arbors: These types of arbors are extremely traditional and feature a romantic archway in the middle.
Pergolas: For an arbor with a more rectangular silhouette, choose a pergola! Traditionally, these consist of four rounded columns supporting a slatted roof.
Latticed Arbors: A latticed arbor is made of criss-crossed, thin pieces of wood---perfect to support crawling vines and flowers growing over the arbor itself.
Gabled Arbors: Very similar to an arched arbor, a gabled arbor simply has a peak at its top instead of a rounded curve.