Many windows have some degree of online documentation in the form of a Help system that you can access by clicking the small question mark icon in the upper-righthand portion of the screen. The help is context-sensitive and will be relevant to the window from which you've accessed it.
In addition, you can press F1 at almost any time to display help. In some situations, pressing F1 will display only a tiny yellow message (known as a tool tip) with a brief description of the item with the focus; at other times, F1 will launch an online index of help topics. Sometimes F1 will have no effect whatsoever.
Furthermore, if you hold the pointer over many screen objects (such as a window's toolbar), a tool tip may appear. A tool tip may display nothing more than the name of the object to which you're pointing, but in other cases, it may provide additional information. For example, placing the pointer on the system clock pops up the date. You can turn tool tips off in the Windows interface by going to Control Panel Appearance and Personalization Folder Options View and turning off the option "Show pop-up description for folder and Desktop items." Note that this won't necessarily turn off tool tips in other applicationsonly Explorer.