In the right side of the menubar there are a lots of more ore less usefull apps, like time machine, airport, bluetooth, ... (coming with Mac OS X) and 3rd party apps like skitch, dropbox, caffeine. At the rightmost side there are the menubar apps coming with Mac OS X. These apps are also called Ã¢??menu extras". The menu extras you can reposition (by CMD+click), so that the most usefull apps are visible all the time (except you have a very small screen or an app with a very large menu). 3rd party apps can't be repositioned. in that way Their position at the menubar depends on the launch order of the apps.
Depending on what's the size of your screen, the space between app menu and menubar apps is sufficient to see all menubar apps or not. With a giant display of an 27" iMac or a large display of an 17' MacBookPro the menubar normally is no problem, because the screen is so large that there is every time enough space between app menu and menubar apps. But if you have a 13" Macbook (Pro | Air) with a resolution of 1280x800, most time some of your menubar apps are hidden by the app menu. And with Murphy's law: If you want to use a menubar app, exactly this app is hidden by the app menu. That sucks.