A window that is prominent and juts out beyond the wall of the house usually does so for decorative rather than practical reasons. They do add to the attraction of the design. However, it can be that such windows are bringing extra light into the house, important in itself.
A Bay window is curved and therefore has to protrude because the house's wall is straight. Bay windows are actually a combination of windows often with a stationary window in the middle. Then there are Dormer windows. These are vertical windows protruding through a sloping roof. It's called a gable dormer if it has its own gable.
Bay windows are easy to spot on a house. They stick out a few feet from the rest of the exterior wall of the house. Inside, the bay window usually has a seat near the window. These windows are popular because they allow for the residents to be able to sit in the window and be able to look out for a nice view.
Sometimes, there is not a seat in the bay window because it has a table scooted up to the window so more than one person can experience and enjoy the view. Most bay windows are square in shape or have several angles. However, they would have three sides to the part of the window that sticks out.