A gazebo is an outdoor structure with a (most often pointed)
roof and no full-length side inclosures. It is usually round, though many models are
hexagonal.
Gazebos (or gazeboes, either can be used as a plural form) were originally built so contractors, landscapers and homeowners could survey a property's landscape and supervise a construction or landscaping project. The structure, with its roof and open sides, allowed people to see what was going on without being subject to the weather. The open sides allow for communication between the supervisor and workers. The word is believed to have been derived from the word 'gaze', as people needed to be able to 'gaze' at the landscape or the work that took place there.
Gazebos can range greatly in size and decorative style. Many contemporary models are anywhere between six feet and sixteen feet in diameter. While gazebos of old served a mainly practical purpose, today's models are generally used to create a visual effect in a yard, park or other "green" area.
Some of the models advertised on the internet are described as "rustic," "unique" or "detailed." Appearance, though an important element during the 19th century, did not originally take precedence over a gazebo's ability to provide shade and keep the lines of communication open. They began to become more ornate during the early 20th century.
The main use of a modern gazebo is to provide shade and to enhance the area's visual appearance. Gazebos also make popular wedding locations; they allow for outdoor weddings while providing relief from the sun.
Most gazebos sold today (or most of those sold online, anyway) are made of wood. Some of the larger contemporary models have glass windows and are held together with metal as opposed to wood. The most famous "glass gazebo" is most likely the one on the grounds of Austria's Castle Hellbrunn, which was featured prominently in The Sound of Music.
Gazebo assembly kits generally cost between $1500 and $5000 (US). Prices vary depending on the size of the gazebo and the amount of detailing. As is the case with other such structures, the presence of a gazebo on one's property may help increase the property value.
Resources:
http://www.gazebos.com (no, really)
http://dictionary.reference.com/search?q=gazebo
"Your Sound of Music Keepsake" MM-Verlag Ges. 1993.