Login Items allows you to set applications to start automatically when you first log in to your Mac. They are used by many apps to run supporting apps in the background. You may have some items there added by apps and not realize it. You can also add your own apps. It is important to understand what the checkboxes next to the apps in the list do. If you want to stop an app from automatically launching, unchecking it won't do that. You need to remove the app from the list.
If you take a look under System Preferences and you go to Users and Groups, select your current User there, you'll see two tabs here. Your Password tab and Login Items.
Under Login Items you'll see a list of things that will automatically start. These are apps that automatically start when you start your Mac or, more likely, when you log into this user account. Almost everybody is going to see something called iTunes Helper which is basically part of iTunes that runs in the background.
You may see some other apps depending upon what you've installed. A lot of times things added here aren't things that you specifically added manually on your own but when you installed an app it put something here.
Sometimes if you're having trouble like you notice that there's an app that always seems to startup and you don't want it to start up when you login this is where to go to get rid it.
Now one important thing to note is how these check boxes work here on the left because this is what people misunderstand most about the login items. If you uncheck it, it does not mean that it does not launch. So I get people saying hey, I unchecked something and it's still launching when I login.
Notice this column is called Hide. So when it says, even here at the bottom, to hide an application when you log in select the checkbox in the Hide column next to the application. So when I put a check there the application launches, it launches hidden. If I don't have a check there then the application launches but it launches in front.
So, for instance, if I wanted to always have let's say Pages startup every time I started up my Mac because I'm a writer and that's what I go to, I can add Pages here and now Pages will automatically launch when I log into this account and it will appear. If I check it Pages will launch but it will be hidden and I just bring it to the front.
So if you want to really get rid of something, like you notice that Pages is launching for some reason and you don't remember adding it here, you hit the minus button to remove it from the list. It's the only way to stop it from automatically launching.
Always leave iTunes Helper running, you know don't worry about that. That runs in the background all the time and it's running on every Mac. Then anything else you've got running here, try to figure out why it's there because it may be some app that you use and there's a component of it that needs to run. So removing something may not be what you want to do.
It's good to know about this menu, know about how it works, and know how to add items here if you want, remove items, and how exactly this checkbox works.