In the Mac operating system, the term "menu bar" refers to the the horizontal bar at the top of the screen that generally has a little picture of an apple on the left end, the time of day on the right and words like "File", "Edit", "Format" and so on in-between. Found it? Good, because understanding how this bar works is essential to getting the best out of your Mac.
In this bar, there are two constants: one is the apple, on the far left, and the other is Spotlight on the far right. Clicking the Apple logo on the left gives you access to all the vital functions of your computer, like Sleep, Restart, and Shut Down, as well as the "About This Mac" window and the option to check for software updates.
Save for the apple, the left side of this bar will change depending on which application you have selected. For you switchers, it's similar to the bar that is found at the top of each window in Windows, but cuts down on the space each window occupies by having all applications use the same menu bar. The drop down menus perform the same functions as Windows' menus, but like Windows, will be different depending on which application you currently have selected.