In the US the word 'marquee' is not often used to describe a temporary outdoor structure, everything from scout tents to huge two-story temporary buildings are described as one thing -if it is being used for a party then it is a party tent.
Most of the world follows the US except in the UK and other parts of Europe there has developed a distinction between the two terms:
'party tent' is used for structures at the lower end of the market, often made from PE fabric and designed with the domestic market in mind.
'marquee' is used for the higher end structures suitable for weddings and larger functions.
There remains a substantial middle area of crossover with heavier duty tents and cheaper marquees making the distinction difficult to manage.
Gazebos and canopies are universally smaller often folding structures designed primarily at the domestic consumer. These are designed with convenience prioritised over durability, they can be erected in a matter of seconds and due to the clever folding design are put away with the roof in place.
Pagodas (also known as witches hats or Chinese hats) can be of variable quality but are distinguished by the pointed shape of the roof. Often used to make dramatic entrance areas in to larger party tents or marquees these can be seen as the commercial equivalent of gazebos and canopies.
If you are planning a large formal function with an attractive entrance then in the US ask for a large party tent with pagoda entrance but in the UK ask for a large marquee with pagoda entrance!
If you're after purchasing an outdoor structure then DIY Marquees offers both marquees and party tents for sale as well as offering advice on organising your event and an interactive planning tool to plan your desired layout. http://weddingtentforsale.weebly.com/