A portable application (portable app), sometimes also called standalone, is a computer software program designed to run independently from an operating system. This type of application is stored on a removable storage device such as a CD, USB flash drive, flash card, or floppy disk – storing its program files, configuration information and data on the storage medium alone.
(above text is taken from Wikipedia)
No need to install all software on your system. You can keep and run them from usb or directory of your system by running exe file of that application.
Many applications are available in portable format like....Mozilla, Chrome, gtalk, yahoo messenger. List is very long just mentioned few of them.
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