As already noted above multiple windows are not supported by iOS. This is a major weakness compared to a "real" computer. On the other hand, the extremely limited multi-tasking capabilities of the iPad enables Apple to provide a very smooth user experience by limiting the demands on what amounts to a relatively underpowered system. Furthermore, with a 9.7" screen, Apple believes (and is largely correct) that trying to squeeze multiple windows on a single screen would not be a very satisfactory user experience.