An arrangement is when flowers are arranged in a base of some kind. More sustainable ways can involve using chickenwire to stablise your florals, this helps make a a more freeform and natural arrangement. Other ways include the use of floral foam.
There are many types of arrangements to suit all needs from symmetrical to asymmetrical, very compact and full arrangements to more styled and minimal. Two standard types of arrangements are your table arrangement (visible on all sides, best to go on the centre of a table) or your wall-facing arrangement (visable on three sides, best placed on a shelf or side table)
Below are examples of styles and types of arrangements.