installing application packages on ubuntu usb

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http://askubuntu.com – I'm new to ubuntu. I downloaded the newest version on a usb stick then I booted my PC with the usb. upon trying out ubuntu I tried installing additional packages/applications, which worked fine. BUT when I then after tried using my ubuntu usb, the new installed apps were not there! if I get that right, ubuntu would only install new packages to the actual PC I'm using it on. not within ubuntu itself!? is there a way that I can install new applications to my ubuntu usb and it should stay within ubuntu so I can use it for other pc's which don't have an internet connection necessary to update/ins (HowTos)