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. You can tell which application you're in by looking at the space to the right of the menu bar apple, where it will say your current program's name (Safari, Mail, Firefox, or iPhoto depending on what window you've most recently clicked). The Preferences settings for the application, where you can set the time for Safari to clear cookies, the default font for TextEdit, and so on, are in this drop down menu.
The right side of the menu bar is less mutable, but much more easily cluttered. For switchers, this part is a sort of cross between Windows' Quick Launch bar and the notification bar. The second constant in the menu bar is found on the far right: a little magnifying glass. Clicking it pops up a search field, giving you access to Spotlight, the program that allows you to search your whole computer.
The rest of the space on this side is where you will find little icons, or "menu extras", such as the system clock. The menu extras can vary, but usually you'll you'll find icons for the volume, wireless internet access, Bluetooth setup and the like. A few of the icons will change depending on their status; for example, the volume icon will have sound waves next to it when sound is on, and the Wi-Fi icon will gray out some of its bars when reception is bad. These native menu extras can be rearranged by clicking and dragging while holding down the Command key, or you can drag them off the bar to make them disappear completely in a puff of smoke (to make them reappear, you'll have to go into the relevant section of System Preferences and recheck "Show [menu extra] in menu bar").
Sometimes you might install applications that will add more menu extras to the fray. You cannot rearrange or delete these with the Command key, but if you want them off, the "Show [menu extra] in menu bar" check box can usually be found in each application's Preferences menu.
That's all for the menu bar! I hope this was helpful.