(I can't belive how much time I've just spent researching this... )
"Orangery appears to be a Victorian word, and Vanbrugh and Hawksmoor's great
orangery at Kensington Palace was called by contemporaries a greenhouse. What
we now call a greenhouse was called a stove until the early nineteenth century,
and the conservatory, with its connotation of romance, is altogether a late
term not in common use until the 1820s." says the Garden History Journal.
The key thing with an orangery is that each citrus tree is placed in front of a very large sash window reaching all the way down to the grown. In summer, the sash window is opened, and in winter, it's closed. The trees themselves are in large containers, and might be pushed outside in a good summer. The orangery is primarily used for citrus growing, so it's usually narrow and long. It may be next to the house, or it may be some distance away, as at Blickling Hall in Norfolk.
In order for 18th century buildings to have this comparatively large expanse of glass, they needed to have wide brick or stone pillars between each sash window.
So an orangery will probably be long and narrow, have a solid back wall, a solid roof, and brick / stone piers interspersed with
Here's a couple of genuine orangeries:
www.homecitrusgrowers.co.uk/citrusplaces/blicklinghall.html
www.homecitrusgrowe rs.co.uk/citrusplaces/hanburyhall2.html
A conservatory is a 19th century concept, reflecting both the technological innovations in working with glass and iron in the C19, the wealth of new plant introductions, AND, crucially, the very high levels of pollution in the atmosphere, which made it difficult to grow plants outside in the Victorian cities. Conservatories are largely glass, held together in an iron structure - like the Palm House at Kew, or the Crystal Palace.
If you look at Amdega's 'orangeries', you'll see they don't make any attempt (as far as I could see) to reflect the historical development of orangeries, and so are just conservatories by a different name.
Still want to know more?
Here's an entire history of greenhouses etc.
www.amazon.co.uk/Glass-Houses-Greenhouses-Conservatories-Orangeries/dp/1854101137/ref=s r11?ie=UTF8&s=books&qid=1279267653&sr=8-1