How to sync only 1 app.

It means they will be erased from the iPhone if they are there...they will still be on your computer to select later to sync to the phone if you decide to do so.

If you have chosen Selected Applications under the Applications tab for your iPhone sync preferences, deselecting an app in the window below followed by a sync will remove the application from your iPhone. Selecting an app in the window below followed by a sync installs the app on the iPhone.

If you are transferring an iTunes playlist to your iPhone, removing music from the playlist in iTunes followed by a sync will remove the music from your iPhone. Adding music to the playlist in iTunes followed by a sync will add the music to your iPhone.

If the apps are already on your iPhone, they don't need to be synced again and aren't synced again when adding an additional app.

Don't change anything with the selected apps under the Applications tab for your iPhone sync preferences and nothing will be touched. If you add an additional app via iTunes and want to install the app on your iPhone, the new selected app will be installed only with the existing apps not touched.

If the box is checked and the app is already on the phone, it won't be synced or removed, it will just be left alone.

If you want to add just one app, the apps list in iTunes should have checkmarks next to every app that is already on the iPhone, and also next to just that one app. Then, just that app will be transferred. The same logic applies to playlists, etc.