When you turn on your Mac, various apps, add-ons (such as menu extras), and invisible background processes open by themselves. Usually these automated actions are exactly what you want, but you may sometimes see items running—either visibly or according to a listing in Activity Monitor (located in /Applications/Utilities)—that you don't recall adding yourself. Where do they come from? Because such items can increase your Mac's startup time (and may decrease its performance), you'll want your machine to load only items that are useful to you. Here's a quick primer on the various kinds of startup and login items and how to manage them.
Open the Users & Groups pane of System Preferences and click the Login Items tab, and you'll see a list of apps (and even files and folders) that open every time you log in. (This list is different for each user account on your Mac.) More often than not, items appear in this list because apps added them to it. Most apps that do so ask you for permission first or offer an 'Open at Login' checkbox for you to check, but not all are so well behaved. In any case, you can add an item to the list manually by clicking the plus sign (+) button, or remove an item by selecting it and clicking the minus sign (-) button.
Earlier versions of OS X relied on two folders—/Library/StartupItems and /System/Library/StartupItems—to hold items designated to load when you start your Mac. Apple now discourages the use of startup items, but some programs (mostly older apps) still use this mechanism. Normally your /System/Library/StartupItems folder should be empty; but if it contains something that you don't use anymore, you can drag the unwanted item to the Trash to prevent it from loading automatically the next time you start your Mac.
Launch daemons and agents
Since OS X 10.4 Tiger, Apple has given developers another mechanism for launching items automatically: launch daemons and agents, controlled by the launchd process. This approach provides more flexibility than either login items or startup items, but it is less transparent to users.
Behind the UNIX curtain: Instead of opening apps directly, launchd loads specially formatted .plist documents (XML preference files) that specify what should launch and under what circumstances. Sometimes these launch items run constantly in the background, sometimes they run at scheduled intervals, and sometimes they run as needed—for example, in response to an event such as a change in a certain file or folder—and then quit.
The .plist files that launchd uses can occupy any of five folders, and their location determines when the items load and with what privileges:
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