A window can be the focal point of a room, and the way it is dressed can help to set the style for the entire room. Your choices determine a lot of what goes on later as windows are a large part of the foundational setting.
What makes a fancy or elaborate style? Cost is an important factor. Simple window dressings usually use much less fabric, are easier to create, and keep the costs lower. A simple fabric shade–either roller style or unlined cotton hung on a pole using rings or ties will be fairly economical. A large window for an opulent setting in a large room will need lots of yardage and may have additional hangings or accouterments to make it fill the scene. Sumptuously dressed windows can have swags and tails, generous drapes, fancy valances and tiebacks.
Curtains and drapes help to frame a view or hide a bad view. They can soften a window, add drama and texture; insulate a window, create quiet and act as a focal point. They can be hung high to provide the illusion of height. The ends can also drape into a puddle on the floor creating richness. Curtains can be used in other ways–as room dividers or to define a space, to camouflage a bad wall or as a cover up to messy shelving or even as a screen for dust.