As "hardscape" elements of a garden or park, both a trellis and an arbor are permanent architectural structures that support climbing plants and provide shade or privacy. However, a trellis usually sits against a wall as a single, flat, latticed surface like an extension of a fence. An arbor resembles a covered patio or pergola. It's like an outdoor room with at least four columns supporting an open roof of crossbraces, lattice, or canvas.
Depending on your garden's needs, a trellis or an arbor can add some visual diversity with an otherwise flat lawn or rectilinear hedge. Usually, a landscape designer will choose a trellis to add height to walls, increase privacy, separate different parts of the garden with a spacer that's permeable to air and light, or specifically to support a vine that cannot grow upright on its own.
A trellis looks like a decorative fence: a few aligned posts joined with latticework or other cross pieces. It can attach to an existing structure or independently mounted with a poured cement base. Trellises made of wood, such as teak or cedar, will naturally weather and look more organically integrated with the rest of your landscape. For those wanting a more colorful finish, polyvinyl and metal offer another possible material. Although a trellis can take the place of a wall, hedge, fence, or tree, most people choose to grow a climbing plant such as a rose, camellia, or honeysuckle.
An arbor, on the other hand, takes up three dimensions and usually spans a larger area. People use arbors, sometimes called pergolas, to cover a paved walkway with a series of integrated arches, roof an outdoor living space like a patio or deck, or frame a seating area such as a bench or swing. This massive structure has more elaborate vertical supports, called colonnades, to hold up a roof of lattice, slats, or even retractable canvas.
Not only does an arbor add a focal point for a plain backyard, but it brings the indoors outside by providing denser shade without sacrificing air circulation. Some plants require a strong support system that won't buckle under twisting vines, such as wisteria, grapes, and jasmine. These hefty plants will eventually grow to a thick canopy of fragrant flowers and fluttery leaves. Smaller arbors could cover a porchless entryway, short staircase, or birdbath, to ease the transition between different parts of your estate.