Very useful interface objects for

• distinguishing between different applications
• within an application, for distinguishing different types of information that is being conveyed to the user or requested from the user
• for interrupting users with warnings and confirmation requests

• While tiled windows are more easy for beginners to understand and manipulate, overlapping windows are far more flexible.
• It is important to signal to the user which window is on top and which window is below.
• Partial occlusion already does a good job in this respect (text in windows should be surrounded by whitespace that is at least one character broad).
• Additional support through 3D effects (e.g., slanted edges, brighter and darker sides, shadows, delayed expansion and shrinking)